Add flakes as experimental feature to Nix config

[sbernauer]

Fixes

trino-operat… │ Running cmd: make regenerate-nix && nix-build . -A docker --argstr dockerName "${EXPECTED_REGISTRY}/trino-operator" && ./result/load-image | docker load
trino-operat… │ make[1]: Entering directory '/home/sbernauer/stackabletech/trino-operator'
trino-operat… │ nix run -f . regenerateNixLockfiles
trino-operat… │ error:
trino-operat… │        … while calling the 'derivationStrict' builtin
trino-operat… │ 
trino-operat… │          at /derivation-internal.nix:9:12:
trino-operat… │ 
trino-operat… │             8|
trino-operat… │             9|   strict = derivationStrict drvAttrs;
trino-operat… │              |            ^
trino-operat… │            10|
trino-operat… │ 
trino-operat… │        … while evaluating derivation 'regenerate-nix-lockfiles'
trino-operat… │          whose name attribute is located at /nix/store/gm0k19k4131pblz3pji7dzbc27zm0pv0-nixpkgs-src/pkgs/stdenv/generic/make-derivation.nix:331:7
trino-operat… │ 
trino-operat… │        … while evaluating attribute 'text' of derivation 'regenerate-nix-lockfiles'
trino-operat… │ 
trino-operat… │          at /nix/store/gm0k19k4131pblz3pji7dzbc27zm0pv0-nixpkgs-src/pkgs/build-support/trivial-builders/default.nix:103:16:
trino-operat… │ 
trino-operat… │           102|       ({
trino-operat… │           103|         inherit text executable checkPhase allowSubstitutes preferLocalBuild;
trino-operat… │              |                ^
trino-operat… │           104|         passAsFile = [ "text" ]
trino-operat… │ 
trino-operat… │        error: attribute 'fetchTree' missing
trino-operat… │ 
trino-operat… │        at /nix/store/xnxmzwkgf4vnq2vhivk7mzrg3vmsasqw-crate2nix-src/crate2nix/default.nix:6:15:
trino-operat… │ 
trino-operat… │             5|     in
trino-operat… │             6|     import "${builtins.fetchTree flakeLock.nodes.nixpkgs.locked}" { }
trino-operat… │              |               ^
trino-operat… │             7|   )
trino-operat… │ make[1]: *** [Makefile:147: regenerate-nix] Error 1
trino-operat… │ make[1]: Leaving directory '/home/sbernauer/stackabletech/trino-operator'

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