Share build_provider_cmd in the Makefile

[t0yv0]

build_provider_cmd is generalized to be a function taking the destination path and performing all platform-specific and platform local builds of the provider in the bridged provider Makefile.

Specifically:

build_provider_cmd receives $(1) that expands to the target path for the binary, such as bin/pulumi-resource-random.
It also receives the environment GOOS and GOARCH specifying which platform to build for.
If those are not specified it should build for the default platform.

Previously the go provider builder was duplicated across provider: and darwin-amd64/$(PROVIDER) targets.

This is intended to be used in providers such as pulumi-awsx to override the recipe for building the target-specific binary in one place and provide a recipe for building a Node-based binary.

[t0yv0]

This change is part of the following stack:

• Share build_provider_cmd in the Makefile #1280 ◀
  • Customizable build_provider_cmd #1281

_{Change managed by git-spice.}