safely check for standalone interpreter sentinel files

[hrfuller]

PR Checklist

Fixes #746
Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
  Tests omitted because we don't have a builtin interpreter which is not of the standalone interpreter variety.
  Unfortunately we can't unit test these bzl files that use the rctx.
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature (please, look at the "Scope of the project" section in the README.md file)
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Fixes regression to using custom python_interpreter_target

Does this PR introduce a breaking change?

• Yes
• No

Other information

[groodt]

LGTM