fix: build plugin incorrectly combines stdout&stderr

[garethjevans]

What this PR does / why we need it

Which issue(s) this PR fixes

Fixes #3907

Describe testing done for PR

• check out local plugin project
• go clean -modcache
• make build-local
  (builds successfully where as would previously fail)

Release note

Builder plugin no longer combines stdout & stderr when reading the plugin descriptor.

[codecov]

Codecov Report

Merging #3909 (237be80) into main (65ded46) will decrease coverage by 0.93%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #3909      +/-   ##
==========================================
- Coverage   46.94%   46.01%   -0.94%     
==========================================
  Files         402      427      +25     
  Lines       40243    41794    +1551     
==========================================
+ Hits        18894    19230     +336     
- Misses      19600    20794    +1194     
- Partials     1749     1770      +21     

Impacted Files	Coverage Δ
...oller-manager/controllers/v3_cascade_controller.go	61.90% <0.00%> (-11.43%)	⬇️
packageclients/pkg/packageclient/package_update.go	83.57% <0.00%> (-1.43%)	⬇️
cmd/cli/plugin/cluster/delete.go	12.50% <0.00%> (ø)
cmd/cli/plugin/cluster/get_machinehealthcheck.go	11.42% <0.00%> (ø)
cmd/cli/plugin/cluster/list.go	11.36% <0.00%> (ø)
cmd/cli/plugin/cluster/set_node_pool.go	14.63% <0.00%> (ø)
...cluster/delete_machinehealthcheck_control_plane.go	16.66% <0.00%> (ø)
cmd/cli/plugin/cluster/credentials_update.go	9.23% <0.00%> (ø)
cmd/cli/plugin/cluster/get.go	6.27% <0.00%> (ø)
cmd/cli/plugin/cluster/osimage.go	100.00% <0.00%> (ø)
... and 17 more

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[marckhouzam]

A little late to the party, but I wanted to say thanks for this fix! This bug has been causing problems to many different people and we had trouble tracking it down. Basically, this PR will fix the error many have seen intermittently of:

✖  🐼 - building plugin "cmd/cli/plugin/tkr" failed - invalid character 'F' looking for beginning of value

It turns out that the tkr plugin prints error messages if an unreachable cluster is configured for tanzu.
Now that the builder plugin ignores error printouts, the build will no longer fail when this happens.

So, again, thanks for this fix @garethjevans 🎉

[garethjevans]

no problem @marckhouzam