Force GOOS to linux

[wayt]

What this PR does / why we need it:

Since #4088 ingress-nginx is no longer build in a docker container (linux).
Which break the build in non linux environment:

make build
k8s.io/ingress-nginx/internal/runtime
# k8s.io/ingress-nginx/internal/runtime
internal/runtime/cpu.go:37:21: undefined: cgroups.FindCgroupMountpoint
make: *** [build] Error 2

the cgroups library here is only build on linux vendor/github.com/opencontainers/runc/libcontainer/cgroups/ => // +build linux

This change force go to build the binary for a linux environment using go cross platform build.

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #

Special notes for your reviewer:

Cc @ElvinEfendi

[k8s-ci-robot]

Hi @wayt. Thanks for your PR.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[aledbf]

/ok-to-test

[codecov-io]

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@@            Coverage Diff            @@
##             master    #4113   +/-   ##
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  Coverage          ?   57.76%           
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  Files             ?       87           
  Lines             ?     6459           
  Branches          ?        0           
=========================================
  Hits              ?     3731           
  Misses            ?     2296           
  Partials          ?      432

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[aledbf]

/lgtm
/approve

[aledbf]

@wayt thanks!

Edit: apologies for the issue introduced by the refactoring

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, wayt

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• OWNERS [aledbf]

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[aledbf]

/retest