Use python.envFile to get env file name when running Python code

[DonJayamanne]

Fixes #9691

[amunger]

could these be unit tests? Doesn't seem to requre vscode to be running

[DonJayamanne]

I rely on vscode here as I'm using the VS Code API to create the file watcher and i don't want to mock that, too much work and not worth it at all.

Here we have 6 test and everything runs in <1s.

[DonJayamanne]

@amunger looks like i'll have to create unit tests and mock the vscode classes, as the file change detection doesn't work on CI, possibly due to a large number of files or other.

[codecov-commenter]

Codecov Report

Merging #10876 (0f0fddd) into main (b96c58e) will increase coverage by 0%.
The diff coverage is 89%.

❗ Current head 0f0fddd differs from pull request most recent head d524656. Consider uploading reports for the commit d524656 to get more accurate results

@@          Coverage Diff           @@
##            main   #10876   +/-   ##
======================================
  Coverage     63%      63%           
======================================
  Files        486      489    +3     
  Lines      33791    33863   +72     
  Branches    5502     5508    +6     
======================================
+ Hits       21408    21492   +84     
+ Misses     10342    10327   -15     
- Partials    2041     2044    +3     

Impacted Files	Coverage Δ
src/platform/common/variables/types.ts	100% <ø> (ø)
...ommon/process/environmentActivationService.node.ts	72% <75%> (-1%)	⬇️
...riables/customEnvironmentVariablesProvider.node.ts	83% <90%> (+10%)	⬆️
src/kernels/raw/finder/jupyterPaths.node.ts	79% <100%> (+1%)	⬆️
.../kernels/raw/launcher/kernelEnvVarsService.node.ts	81% <100%> (+<1%)	⬆️
src/kernels/variables/preWarmVariables.node.ts	91% <100%> (+<1%)	⬆️
src/platform/common/process/processFactory.node.ts	90% <100%> (ø)
src/platform/common/net/httpClient.ts	77% <0%> (-12%)	⬇️
src/kernels/installer/productInstaller.node.ts	74% <0%> (-5%)	⬇️
src/kernels/raw/launcher/kernelProcess.node.ts	69% <0%> (-1%)	⬇️
... and 63 more