title: Gatsby Build fails on Netlify by works locally. Ffmpeg dependency issue
body: <p>I'm trying to deploy my site on Netlify, and the <code>gatsby build</code> command works perfectly fine on my system. But when I run in on Netlify, I run into the issue shown below.</p>
  
  <p>Unhandled rejection: cannot find ffprobe seems to be the issue. I think it's because I have ffmpeg installed on my system (macOS) but there is no installation procedure during the build process in Netlify. Can somebody please guide me as to what I can do to fix this?</p>
  
  <pre><code>1:19:18 AM: info bootstrap finished - 5.299 s
  1:19:18 AM: ⠀
  1:19:22 AM: error UNHANDLED REJECTION Cannot find ffprobe
  1:19:23 AM: 
  1:19:23 AM:   Error: Cannot find ffprobe
  1:19:23 AM:   
  1:19:23 AM:   - ffprobe.js:145 
  1:19:23 AM:     [repo]/[fluent-ffmpeg]/lib/ffprobe.js:145:31
  1:19:23 AM:   
  1:19:23 AM:   - capabilities.js:194 
  1:19:23 AM:     [repo]/[fluent-ffmpeg]/lib/capabilities.js:194:9
  1:19:23 AM:   
  1:19:23 AM:   - async.js:52 
  1:19:23 AM:     [repo]/[async]/lib/async.js:52:16
  1:19:23 AM:   
  1:19:23 AM:   - async.js:1209 
  1:19:23 AM:     [repo]/[async]/lib/async.js:1209:30
  1:19:23 AM:   
  1:19:23 AM:   - capabilities.js:186 
  1:19:23 AM:     [repo]/[fluent-ffmpeg]/lib/capabilities.js:186:13
  1:19:23 AM:   
  1:19:23 AM:   - capabilities.js:123 
  1:19:23 AM:     [repo]/[fluent-ffmpeg]/lib/capabilities.js:123:9
  1:19:23 AM:   
  1:19:23 AM:   - async.js:52 
  1:19:23 AM:     [repo]/[async]/lib/async.js:52:16
  1:19:23 AM:   
  1:19:23 AM:   - async.js:1209 
  1:19:23 AM:     [repo]/[async]/lib/async.js:1209:30
  1:19:23 AM:   
  1:19:23 AM:   - capabilities.js:116 
  1:19:23 AM:     [repo]/[fluent-ffmpeg]/lib/capabilities.js:116:11
  1:19:23 AM:   
  1:19:23 AM:   - utils.js:223 
  1:19:23 AM:     [repo]/[fluent-ffmpeg]/lib/utils.js:223:16
  1:19:23 AM:   
  1:19:23 AM:   - which.js:68 F
  1:19:23 AM:     [repo]/[which]/which.js:68:16
  1:19:23 AM:   
  1:19:23 AM:   - which.js:80 E
  1:19:23 AM:     [repo]/[which]/which.js:80:29
  1:19:23 AM:   
  1:19:23 AM:   - which.js:89 
  1:19:23 AM:     [repo]/[which]/which.js:89:16
  1:19:23 AM:   
  1:19:23 AM:   - index.js:42 
  1:19:23 AM:     [repo]/[isexe]/index.js:42:5
  1:19:23 AM:   
  1:19:23 AM:   - mode.js:8 
  1:19:23 AM:     [repo]/[isexe]/mode.js:8:5
  1:19:23 AM:   
  1:19:23 AM: 
  1:19:23 AM: not finished run queries - 4.168s
  1:19:23 AM: not finished Generating image thumbnails - 4.111s
  1:19:23 AM: (sharp:1520): GLib-CRITICAL **: 16:19:23.397: g_hash_table_lookup: assertion 'hash_table != NULL' failed
  1:19:23 AM: (sharp:1520): GLib-CRITICAL **: 16:19:23.438: g_hash_table_lookup: assertion 'hash_table != NULL' failed
  1:19:23 AM: Skipping functions preparation step: no functions directory set
  1:19:23 AM: Caching artifacts
  1:19:23 AM: Started saving node modules
  1:19:23 AM: Finished saving node modules
  1:19:23 AM: Started saving pip cache
  1:19:23 AM: Finished saving pip cache
  1:19:23 AM: Started saving emacs cask dependencies
  1:19:23 AM: Finished saving emacs cask dependencies
  1:19:23 AM: Started saving maven dependencies
  1:19:23 AM: Finished saving maven dependencies
  1:19:23 AM: Started saving boot dependencies
  1:19:23 AM: Finished saving boot dependencies
  1:19:23 AM: Started saving go dependencies
  1:19:23 AM: Finished saving go dependencies
  1:19:27 AM: Error running command: Build script returned non-zero exit code: 1
  1:19:27 AM: Failing build: Failed to build site
  1:19:27 AM: failed during stage 'building site': Build script returned non-zero exit code: 1
  1:19:27 AM: Finished processing build request in 1m15.231125909s
  </code></pre>
  
  <p>My <code>gatsby-config.js</code> file is as follows:</p>
  
  <pre><code>module.exports = {
    siteMetadata: {
      title: "#######",
      author: "#######"
    },
    plugins: [
      "gatsby-plugin-sass",
      {
        resolve: "gatsby-source-filesystem",
        options: {
          name: "images",
          path: `${__dirname}/src/images`
        }
      },
      "gatsby-transformer-sharp",
      "gatsby-plugin-sharp",
      "gatsby-transformer-ffmpeg",
      "gatsby-plugin-ffmpeg"
    ]
  }
  
  </code></pre>