Improve error message when env vars such as CNB_TARGET_OS are not set

[edmorley]

In heroku/buildpacks-procfile#214 the buildpack failed with the following error message:

step-build-and-push [Error: Internal Buildpack Error]
step-build-and-push Couldn't determine target os: environment variable not found
step-build-and-push err:  heroku/procfile@3.0.0 (1)
step-build-and-push ======== Output: heroku/procfile@3.0.0 ========

This was due to the CNB_TARGET_OS env var not being set correctly (when Buildpack API 0.10 says it should), however, we could improve the error message to include the env var name to make such scenarios easier to debug.

libcnb.rs/libcnb/src/error.rs

Lines 26 to 30 in f098399

	#[error("Couldn't determine target os: {0}")]
	CannotDetermineTargetOs(std::env::VarError),
	#[error("Couldn't determine target arch: {0}")]
	CannotDetermineTargetArch(std::env::VarError),