Clarify build environment handling, from sylabs 213

This fixes a subpart of https://github.com/apptainer/apptainer-userdocs/issues/239 by merging

https://github.com/sylabs/singularity-userdocs/pull/213 which fixed
https://github.com/sylabs/singularity-userdocs/issues/15

The original PR description was:

Host env not passed into the build scriptlets. Values can be passed in using new templating support. Re-write SINGULARITY_ENVIRONMENT example, which was less than clear.

apptainer / apptainer-userdocs

Clarify build environment handling, from sylabs 213 #255