libnixf: remove fmt dependency #1926

Job	Run time
nix-build-develop-clang (none, release, true)	2m 56s
nix-build-develop-gcc (none, release, true)	3m 17s
nix-build-develop-gcc (none, release, false)	3m 14s
nix-build-develop-clang (none, release, false)	2m 56s
nix-build-develop-gcc (none, debug, true)	3m 8s
nix-build-develop-clang (none, debug, true)	2m 41s
nix-build-develop-gcc (none, debug, false)	3m 4s
nix-build-develop-clang (none, debug, false)	2m 40s
nix-build-develop-gcc (undefined, release, true)	6m 48s
nix-build-develop-gcc (undefined, release, false)	7m 41s
nix-build-develop-gcc (undefined, debug, true)	3m 42s
nix-build-develop-gcc (undefined, debug, false)	3m 44s
	45m 51s

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