message: doc: remove boostrap info from GUIX_COMMON_FLAGS doc
  Passing ADDITIONAL_GUIX_COMMON_FLAGS="--no-substitutes --bootstrap" as
  suggested doesn't work:
  ```bash
  ...outputting in: '/bitcoin/guix-build-a1f0b8b62eb8/output/x86_64-linux-gnu'
  ...bind-mounted in container to: '/outdir-base/x86_64-linux-gnu'
  guix time-machine: error: bootstrap: unrecognized option
  ```
  and I think bootstrapping is more than covered in the preceding "Choose
  your security model" section.
diff: @@ -205,10 +205,7 @@ find output/ -type f -print0 | sort -z | xargs -r0 sha256sum
  * _**ADDITIONAL_GUIX_COMMON_FLAGS**_
  - Additional flags to be passed to all `guix` commands. For a fully-bootstrapped
  - build, set this to `--bootstrap --no-substitutes` (refer to the [security
  - model section](#choosing-your-security-model) for more details). Note that a
  - fully-bootstrapped build will take quite a long time on the first run.
  + Additional flags to be passed to all `guix` commands.
  * _**ADDITIONAL_GUIX_TIMEMACHINE_FLAGS**_
  