GIT-VERSION-GEN: allow it to be run in parallel #1903

Job	Run time
config	5s
fuzz smoke test	3m 41s
win+VS build	0s
almalinux-8 (almalinux:8)	3m 29s
debian-11 (debian:11)	3m 28s
linux-musl (alpine)	3m 28s
linux32 (i386/ubuntu:focal)	3m 28s
pedantic (fedora)	3m 29s
documentation	3m 39s
linux-asan-ubsan (ubuntu-latest)	3m 40s
linux-gcc (ubuntu-20.04)	3m 39s
linux-gcc-default (ubuntu-latest)	3m 40s
linux-leaks (ubuntu-latest)	3m 40s
linux-reftable (ubuntu-latest)	3m 40s
linux-reftable-leaks (ubuntu-latest)	3m 39s
linux-sha256 (ubuntu-latest)	3m 38s
linux-meson (ubuntu-latest)	3m 40s
linux-TEST-vars (ubuntu-20.04)	3m 39s
osx-clang (macos-13)	3m 45s
osx-gcc (macos-13)	3m 49s
osx-meson (macos-13)	3m 39s
win+VS test	0s
osx-reftable (macos-13)	3m 46s
sparse	1s
static-analysis	1s
win build	1s
win test	1s
	1h 12m 45s

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