Minor improvements in `Makefile` and `authmate.md`. #1543

Job	Run time
Build binaries (ubuntu-22.04, linux, amd64)	44s
Build binaries (ubuntu-22.04, linux, arm64)	29s
Build binaries (macos-14, darwin, amd64)	1m 1s
Build binaries (macos-14, darwin, arm64)	37s
Build Docker image	8m 38s
	11m 29s

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