Automatic merge from submit-queue (batch tested with PRs 49083, 45540, 46862)
Respect KUBE_BUILD_PLATFORMS set by user
**What this PR does / why we need it**:
Currently the only(?) toggle available for custom arch builds is
KUBE_FASTBUILD. We should allow the user to specify a list
of arch(es) in the environment variable KUBE_BUILD_PLATFORMS.
Example:
KUBE_BUILD_PLATFORMS="linux/amd64 linux/arm64" hack/build-cross.sh
**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #
Fixes#20365
**Special notes for your reviewer**:
**Release note**:
```release-note
NONE
```
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