check if have all and other platforms set in the --platform flag

[cpanato]

• check if have all and other platforms set in the --platform flag

../../google/ko/main build --base-import-paths \
--platform=all,linux/arm --tags v1.5.1-96-gd1a4d08-dirty --tags d1a4d086309e22cc85c0c59ac662606a58c81438 github.com/sigstore/cosign/cmd/cosign
Error: validating options: all or specific platforms should be used
2022/03/02 14:12:16 error during command execution:validating options: all or specific platforms should be used
make: *** [ko] Error 1

Address #542 (comment)

/cc @imjasonh

[codecov-commenter]

Codecov Report

Merging #630 (57b55f2) into main (e4a01f6) will decrease coverage by 0.07%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #630      +/-   ##
==========================================
- Coverage   51.47%   51.40%   -0.08%     
==========================================
  Files          44       44              
  Lines        3279     3284       +5     
==========================================
  Hits         1688     1688              
- Misses       1380     1385       +5     
  Partials      211      211              

Impacted Files	Coverage Δ
pkg/commands/options/validate.go	0.00% <0.00%> (ø)

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[github-actions]

This Pull Request is stale because it has been open for 90 days with
no activity. It will automatically close after 30 more days of
inactivity. Keep fresh with the 'lifecycle/frozen' label.

[cpanato]

rebased