use GitHub Actions to cache environments and use dependent CI workflow structure

[zacharyburnett]

Notable changes:

• workflow jobs are defined in GitHub Actions to simplify the environment caching and construction
• all environments are cached to speed up subsequent runs; cache key is set to hash the build files so that if a build requirement changes the environment is built from scratch
• matrices are dependent in a workflow such that a commit that fails style checks will not run tests, and so on
• matrices test a more comprehensive set of platforms and Python versions

Checklist

• Tests
• Documentation
• Change log
• Milestone
• Label(s)

[codecov]

Codecov Report

Merging #58 (584d547) into master (5f86aab) will not change coverage.
The diff coverage is n/a.

❗ Current head 584d547 differs from pull request most recent head 4d2bb02. Consider uploading reports for the commit 4d2bb02 to get more accurate results

@@          Coverage Diff           @@
##           master     #58   +/-   ##
======================================
  Coverage    0.00%   0.00%           
======================================
  Files          25      25           
  Lines        3060    3027   -33     
======================================
+ Misses       3060    3027   -33     

Impacted Files	Coverage Δ
src/stpipe/step.py	0.00% <0.00%> (ø)
src/stpipe/cmdline.py	0.00% <0.00%> (ø)
src/stpipe/pipeline.py	0.00% <0.00%> (ø)

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[zacharyburnett]

failing tests in JWST downstream:

test_shutter_size_on_sky

assert 0.19432383493814478 < 0.194

https://github.com/spacetelescope/stpipe/runs/7154721297?check_suite_focus=true#step:8:111

test_functional_fs_msa[msa]

assert_allclose(v2, ins_tab['xV2V3'])

https://github.com/spacetelescope/stpipe/runs/7154721297?check_suite_focus=true#step:8:312

test_functional_ifu_prism

assert_allclose(v2, ins_tab['xV2V3'])

https://github.com/spacetelescope/stpipe/runs/7154721297?check_suite_focus=true#step:8:669

test_functional_ifu_grating

assert_allclose(v2, ins_tab['xV2V3'])

https://github.com/spacetelescope/stpipe/runs/7154721297?check_suite_focus=true#step:8:870

test_in_slice[1]

assert r[~np.isnan(r)].size == xinv.size

https://github.com/spacetelescope/stpipe/runs/7154721297?check_suite_focus=true#step:8:936

test_in_slice[17]

assert r[~np.isnan(r)].size == xinv.size

https://github.com/spacetelescope/stpipe/runs/7154721297?check_suite_focus=true#step:8:1005

test_nirspec_wcs_roundtrip

assert_allclose(y, yp, rtol=0, atol=3e-4)

https://github.com/spacetelescope/stpipe/runs/7154721297?check_suite_focus=true#step:8:1065

[nden]

Looks good.
Are all Required checks necessary?

[zacharyburnett]

Are all Required checks necessary?

I will update the repository settings to include the current "required" checks, but it's probably only "required" to test Python 3.9 on Linux as the operational environment