Revert "Run docker_credentials_gcr in warmer (#1780)"

[imjasonh]

Description

This reverts commit cf4822c.

I have a theory in #702 (comment) that this change causes the regression in v1.7.0 which also blocks us from cutting new releases.

The original justification in #1780 was that the credential helpers aren't available in multi-arch images, but since #1848 we're go building them ourselves with GOARCH set, so that shouldn't be an issue. @tejal29 any more context on that issue might help us come up with a better solution.

In any case, I don't think that change was necessary, so even if it doesn't fix the bug, we should be able to get by without it. With images being pushed for every commit we can test whether this fixes everyone's issues, and if so, we can cut a 1.8.0. If not, we can at least keep digging for the bug.

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Revert change related to GCR credential helpers

[imjasonh]

Nevermind, I built an image myself at gcr.io/imjasonh/kaniko@sha256:2fad0b62dcf97853b8afc87c57c5b613b27f91d2911d8e5ab2bec8b8daf40e3a and it still exhibits the broken behavior, where :v1.6.0 works for me.

There's a different issue at play here, something between v1.6.0...v1.7.0