[CI] Remove pin for ancient nix version #2659

Job	Run time
check_format	1m 34s
x86_64-gnu-test (1.7.3)	33m 52s
x86_64-gnu-test-preview_mt	5m 33s
x86_64-gnu-test (1.8.2)	36m 31s
x86_64-gnu-test (1.9.2)	33m 56s
x86_64-gnu-test (1.10.1)	34m 55s
x86_64-musl-test	38m 22s
x86_64-gnu-test (1.11.2)	37m 39s
x86_64-gnu-test (1.12.2)	33m 42s
x86_64-gnu-test (1.13.3)	34m 26s
x86_64-gnu-test (1.14.0)	34m 18s
x86_64-gnu-test (1.0.0, FLAGS=-Dwithout_ffi USE_PCRE1=true)	32m 50s
x86_64-gnu-test (1.1.1, FLAGS=-Dwithout_ffi USE_PCRE1=true)	33m 37s
x86_64-gnu-test (1.2.2, USE_PCRE1=true)	36m 52s
x86_64-gnu-test (1.3.2, USE_PCRE1=true)	36m 20s
x86_64-gnu-test (1.4.1, USE_PCRE1=true)	34m 16s
x86_64-gnu-test (1.5.1, USE_PCRE1=true)	34m 10s
x86_64-gnu-test (1.6.2, USE_PCRE1=true)	33m 47s
	9h 26m 40s

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