Failing to delete source archive should not fail GCB builds

[briandealwis]

@dhodun mentioned that an error deleting a source archive from a GCB build failed the overall build.

cleaning up source tar after build: storage: object doesn't exist

This should be logged, but it is not a build failure.

It's unclear how this source archive was deleted. Currently we name the source archives with a randomly-generated ID, so theoretically there could be a collision, though @dhodun was unable to find evidence

This PR makes two changes:

• it logs any deletion errors rather than fail the build
• it uses a UUID to name the source archive to avoid collisions

User facing changes (remove if N/A)

• Google Cloud Builder builds no longer treat errors deleting the source archive as a failure

Follow-up Work (remove if N/A)

[codecov]

Codecov Report

Merging #5891 (cdefb86) into master (ff5038a) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #5891      +/-   ##
==========================================
- Coverage   70.91%   70.89%   -0.02%     
==========================================
  Files         447      447              
  Lines       16908    16909       +1     
==========================================
- Hits        11990    11988       -2     
- Misses       4028     4030       +2     
- Partials      890      891       +1     

Impacted Files	Coverage Δ
pkg/skaffold/build/gcb/cloud_build.go	0.00% <0.00%> (ø)
pkg/skaffold/util/tar.go	50.66% <0.00%> (-5.34%)	⬇️
pkg/skaffold/build/buildpacks/init.go	95.23% <0.00%> (ø)
pkg/skaffold/docker/parse.go	87.14% <0.00%> (+0.95%)	⬆️

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[aaron-prindle]

LGTM

[aaron-prindle]

Also not sure if it is interesting but it seems that cloud_build.go is the only file in skaffold/pkg/skaffold/build/gcb/ that does not have an associated _test.go file. Might make sense to add a tracking issue for more testing here if we see more issues.