perf: do allow `GIT_PERF_*` to be overridden again #23081

Job	Run time
config	7s
fuzz smoke test	37s
win+VS build	0s
win build	4m 37s
static-analysis	4m 9s
sparse	3m 25s
documentation	4m 54s
win+Meson build	7m 21s
osx-meson (macos-13)	37m 9s
osx-clang (macos-13)	29m 2s
osx-gcc (macos-13)	26m 12s
linux-meson (ubuntu:rolling)	11m 9s
linux-musl-meson (alpine:latest)	8m 11s
osx-reftable (macos-13)	34m 10s
linux32 (i386/ubuntu:focal)	9m 59s
pedantic (fedora:latest)	1m 36s
linux-sha256 (ubuntu:rolling)	11m 7s
almalinux-8 (almalinux:8)	9m 43s
linux-asan-ubsan (ubuntu:rolling)	32m 47s
debian-11 (debian:11)	9m 58s
linux-TEST-vars (ubuntu:20.04)	11m 48s
linux-reftable (ubuntu:rolling)	11m 16s
linux-breaking-changes (ubuntu:rolling)	10m 28s
linux-reftable-leaks (ubuntu:rolling)	27m 54s
linux-leaks (ubuntu:rolling)	25m 0s
win+VS test	0s
win test (3)	5m 34s
win test (4)	5m 33s
win test (6)	6m 16s
win test (2)	5m 22s
win test (5)	7m 29s
win test (7)	4m 49s
win test (9)	5m 51s
win test (0)	8m 55s
win test (8)	4m 19s
win test (1)	6m 16s
win+Meson test (3)	11m 22s
win+Meson test (1)	9m 37s
win+Meson test (2)	8m 40s
win+Meson test (4)	11m 24s
win+Meson test (8)	7m 2s
win+Meson test (6)	8m 49s
win+Meson test (5)	9m 2s
win+Meson test (9)	7m 5s
win+Meson test (0)	7m 57s
win+Meson test (7)	7m 42s
	8h 1m 43s

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