forge-startup-asserts: Do not error if git isn't installed #761

Job	Run time
Compile / Get matrix	30s
Compile / Compile using Emacs release-snapshot	1m 5s
Compile / Compile using Emacs 30.2	1m 20s
Compile / Compile using Emacs snapshot	1m 2s
Compile / Compile using Emacs 30.1	1m 17s
Compile / Compile using Emacs 29.4	1m 23s
Compile / Compile using Emacs 29.1	1m 10s
	7m 47s

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