Don't assume env vars are set in model handler

[damccorm]

This broke https://colab.sandbox.google.com/github/apache/beam/blob/master/examples/notebooks/beam-ml/run_custom_inference.ipynb#scrollTo=7f841596-f217-46d2-b64e-1952db4de4e7

since the custom model handler doesn't set _env_vars

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[codecov]

Codecov Report

Merging #29200 (171951e) into master (2f8eebb) will not change coverage.
Report is 1 commits behind head on master.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master   #29200   +/-   ##
=======================================
  Coverage   38.36%   38.36%           
=======================================
  Files         687      687           
  Lines      101747   101747           
=======================================
  Hits        39039    39039           
  Misses      61129    61129           
  Partials     1579     1579           

Flag	Coverage Δ
python	29.98% <0.00%> (ø)

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Files	Coverage Δ
sdks/python/apache_beam/ml/inference/base.py	0.00% <0.00%> (ø)

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[damccorm]

Run Python_Transforms PreCommit

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[damccorm]

R: @jrmccluskey since ritesh is out

[github-actions]

Stopping reviewer notifications for this pull request: review requested by someone other than the bot, ceding control

[jrmccluskey]

LGTM