env: Stringify supplied ExternalPrograms as per its docs

[amyspark]

:waves:

Fixes #10901

[amyspark]

this would have to emit a FeatureNew if you use it

I've added, though not sure at all how those are used. Please tell me if I've put the annotation in the wrong place.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 50.96%. Comparing base (05f4e0d) to head (0a23914). Report is 1 commits behind head on master.

:exclamation: Current head 0a23914 differs from pull request most recent head df2dbd0. Consider uploading reports for the commit df2dbd0 to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #11126       +/-   ##
===========================================
- Coverage   70.29%   50.96%   -19.34%     
===========================================
  Files         219      209       -10     
  Lines       48643    45567     -3076     
  Branches    11557     9436     -2121     
===========================================
- Hits        34194    23222    -10972     
- Misses      12004    20249     +8245     
+ Partials     2445     2096      -349     

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[amyspark]

@xclaesse I've reviewed this again and since there's been an improvement in documentation of ExternalProgram.full_path() I've clarified my case there. It's also easier to handle compared with the new typing structure that's present in the variable conversion logic.

Let me know if there's anything else needed I can do 👍