Developers should not be forced to build for all platforms by
default. We also don't want to copy-and-paste the go invocation for
each new platform.

To address both, the target platform(s) are now configurable via
BUILD_PLATFORMS and additional platforms are only enabled in the Prow
CI.

For now this serves as a test that the source actually compiles for
multiple platforms. Building images for different target platforms is a
different problem.