BUILD: Speed up GoReleaser in PR builds with --single-target --skip=before

[cafferata]

The main bottleneck in the PR CI/CD pipeline is the goreleaser build --snapshot step, which takes ~8m19s out of the total ~10m45s (run 23057743439).

Step	Duration
Unit tests	1m 46s
Build binaries (not tagged)	8m 19s
Rest (setup, cache, etc.)	~10s

Two things make it slow:

1. Cross-compilation for all 8 targets (linux, windows, darwin, freebsd x amd64/arm64). For a PR build, we only need to verify the code compiles.
2. Before hooks (go fmt ./..., go mod tidy, go generate ./...) run every time, but are redundant in CI -- formatting/tidy should already be done, and go generate is covered by the stringer install step.

This PR adds --single-target and --skip=before to the snapshot build, which should bring the GoReleaser step from ~8 minutes down to ~1 minute. The full release build (on tag) remains unchanged.

Fixes #4165

[cafferata]

Results are in from run 23105587930:

Step	Before	After	Difference
Build binaries (not tagged)	8m 19s	39s	-93%
Unit tests	1m 46s	1m 47s	-
Total	~10m 45s	~2m 52s	-73%