title: Should `go build` fail if version in `go.mod` is lower than actual?
body: <p>I have <code>go 1.15</code> specified in the <code>go.mod</code> file.</p>
  <p>The current compiler version is the following:</p>
  <pre><code>➜ go version
  go version go1.14.4 linux/amd64
  </code></pre>
  <p>Yet, <code>go build</code> still successfully builds a binary.</p>
  <p>Is this an expected behaviour? Not sure I can find explanation in the modules description.</p>