clear PMIX_ environment variables in broker

[garlick]

Ensure that the flux pmix client plugin (upmi) clears PMIX_* environment variables after finalize is called, so that these variables don't leak through to Flux jobs.

[codecov[bot]]

Codecov Report

Merging #78 (2296cd0) into main (32cb735) will decrease coverage by 0.51%. The diff coverage is 69.23%.

@@            Coverage Diff             @@
##             main      #78      +/-   ##
==========================================
- Coverage   79.44%   78.94%   -0.51%     
==========================================
  Files          10       12       +2     
  Lines        1231     1363     +132     
==========================================
+ Hits          978     1076      +98     
- Misses        253      287      +34     

Impacted Files	Coverage Δ
src/upmi/pmix.c	76.82% <50.00%> (ø)
src/common/libutil/unsetenv_glob.c	70.00% <70.00%> (ø)

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[garlick]

Thanks for the review! I dropped the dead code and I'll set MWP.