local backend: test shells if unknown

[6543]

currently if we don't know the shell we just assume posix.
this adds a small test, to ensure it is and fail gracefully before doing weird stuff.

Test Conf

skip_clone: true
steps:
  build:
    image: "true"
    commands:
      - echo "building..."

Before

After

[codecov]

Codecov Report

❌ Patch coverage is 70.37037% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 19.82%. Comparing base (e11f110) to head (83503d8).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
pipeline/backend/local/command.go	70.00%	6 Missing ⚠️
pipeline/backend/local/errors.go	71.42%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5570      +/-   ##
==========================================
+ Coverage   19.79%   19.82%   +0.02%     
==========================================
  Files         419      420       +1     
  Lines       39796    39820      +24     
==========================================
+ Hits         7878     7894      +16     
- Misses      31210    31218       +8     
  Partials      708      708              

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[6543]

Base work for #5582

[xoxys]

Nit