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
```