Fix build with multiple GOFLAGS

[yselkowitz]

What type of PR is this?

/kind bug

What this PR does / why we need it:

The tag extraction from GOFLAGS doesn't do anything if -tags is not
present in GOFLAGS. Not only does that mean non-sensical tags are
passed, but because GOFLAGS is a space-separated variable, the build
will fail because the build flags because -tags must be comma-separated.

Which issue(s) this PR fixes:

Special notes for your reviewer:

Does this PR introduce a user-facing change?

NONE

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

[k8s-ci-robot]

@yselkowitz: This issue is currently awaiting triage.

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[k8s-ci-robot]

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[ehashman]

/ok-to-test
/priority important-soon
/sig release

[ehashman]

Current behaviour:

# OK
ehashman@red-dot:~$ GOFLAGS='-mod=vendor -p=8 -tags=yay' gotags="selinux,notest,$(echo "${GOFLAGS:-}" | sed -e 's|.*-tags=\([^-]*\).*|\1|')"
ehashman@red-dot:~$ echo $gotags 
selinux,notest,yay

# Bug
ehashman@red-dot:~$ GOFLAGS='-mod=vendor -p=8' gotags="selinux,notest,$(echo "${GOFLAGS:-}" | sed -e 's|.*-tags=\([^-]*\).*|\1|')"
ehashman@red-dot:~$ echo $gotags 
selinux,notest,-mod=vendor -p=8

New behaviour:

ehashman@red-dot:~$ GOFLAGS='-mod=vendor -p=8 -tags=yay' gotags="selinux,notest,$(echo "${GOFLAGS:-}" | sed -ne 's|.*-tags=\([^-]*\).*|\1|p')"
ehashman@red-dot:~$ echo $gotags 
selinux,notest,yay

ehashman@red-dot:~$ GOFLAGS='-mod=vendor -p=8' gotags="selinux,notest,$(echo "${GOFLAGS:-}" | sed -ne 's|.*-tags=\([^-]*\).*|\1|p')"
ehashman@red-dot:~$ echo $gotags 
selinux,notest,

I think this looks correct, we don't want random goflags accidentally included in build tags.

/lgtm

[ehashman]

(p command with -n in sed: https://www.grymoire.com/Unix/Sed.html#uh-15b)

[sttts]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: sttts, yselkowitz

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/retest