Check the right environment variable names

This uses printenv to get the value of the right environment variable
even though it is not part of POSIX because the alternatives are worse:

Using env does not allow printing an individual variable, grep or sed
could be used but then the regex characters would need to be escaped
and that is extremely hard to safely get right.

Using eval could work but then shell metacharacters would need escaping
and that is extremely hard to safely get right.

Suggested-by: shellcheck
Closes: https://bugs.debian.org/959075
See-also: https://www.shellcheck.net/wiki/SC2157