Don't single-quote SKAFFOLD_GO_GCFLAGS

[briandealwis]

Fixes: #4863
Related to: #4606

Description
We can't wrap the SKAFFOLD_GO_GCFLAGS value in single-quotes. Although the compiler doesn't complain, it doesn't apply the flags and so we end up with:

(dlv) c
> main.hello() /workspace/main.go:17 (hits goroutine(20):1 total:1) (PC: 0x66074d)
Warning: debugging optimized function

Verified with Docker and Buildpacks builder.

User facing changes (remove if N/A)

• SKAFFOLD_GO_GCFLAGS is now properly quoted

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@3c10069). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #4864   +/-   ##
=========================================
  Coverage          ?   71.86%           
=========================================
  Files             ?      356           
  Lines             ?    12218           
  Branches          ?        0           
=========================================
  Hits              ?     8780           
  Misses            ?     2786           
  Partials          ?      652           

Impacted Files	Coverage Δ
pkg/skaffold/build/buildpacks/default_args.go	100.00% <ø> (ø)
pkg/skaffold/docker/build_args.go	66.66% <ø> (ø)

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[gsquared94]

tagging @nkubala for #4467 (comment)

[briandealwis]

We do add environment variables elsewhere in that example for skaffold debug.