Require --sdk_location for Dataflow pipelines running with dev SDKs.

[tvalentyn]

Beam Dev SDK containers also include Beam SDK, which might be not the same as what users have locally when they submit a Dataflow pipeline. Provide an explicit error to make sure pipelines running with Dev SDKs supply an SDK location.

This was already a requirement before we switched to use external containers.

[tvalentyn]

Run Portable_Python PreCommit

[tvalentyn]

Run Python 3.8 PostCommit

[codecov]

Codecov Report

Merging #28670 (c1d13e3) into master (adb1a94) will decrease coverage by 0.25%.
Report is 19 commits behind head on master.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master   #28670      +/-   ##
==========================================
- Coverage   72.20%   71.95%   -0.25%     
==========================================
  Files         684      680       -4     
  Lines      101141   100166     -975     
==========================================
- Hits        73025    72079     -946     
+ Misses      26540    26511      -29     
  Partials     1576     1576              

Flag	Coverage Δ
python	82.53% <66.66%> (-0.22%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
...s/python/apache_beam/runners/portability/stager.py	84.39% <ø> (ø)
...on/apache_beam/runners/dataflow/dataflow_runner.py	81.15% <66.66%> (+0.14%)	⬆️

... and 26 files with indirect coverage changes

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[github-actions]

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[damccorm]

Thanks!