ci(gh-actions): Misc improvements

[PetarKirov]

• ci(gh-actions/check-style): Build the devshell as a separate step
• ci(gh-actions/update-flake-lock): Use DeterminateSystems/nix-installer-action
• fix(scripts/commit_flake_update): Exit early if flake inputs are up to date
• ci(gh-actions): Set the PR description to be the same as the commit msg

[github-actions]

Thanks for your Pull Request!

Below you will find a summary of the cachix status of each package, for each supported platform.

package	x86_64-linux	x86_64-darwin	aarch64-darwin
dmd	✅ cached	✅ cached	🚫 not supported
dmd-2_092_1	✅ cached	✅ cached	🚫 not supported
dmd-2_096_1	✅ cached	✅ cached	🚫 not supported
dmd-2_098_1	✅ cached	✅ cached	🚫 not supported
dmd-2_100_2	✅ cached	✅ cached	🚫 not supported
dmd-2_102_2	✅ cached	✅ cached	🚫 not supported
dmd-2_103_1	✅ cached	✅ cached	🚫 not supported
dmd-2_104_2	✅ cached	✅ cached	🚫 not supported
dmd-2_105_2	✅ cached	✅ cached	🚫 not supported
dmd-binary-2_079_1	✅ cached	✅ cached	🚫 not supported
dmd-binary-2_080_1	✅ cached	✅ cached	🚫 not supported
dmd-binary-2_081_2	✅ cached	✅ cached	🚫 not supported
dmd-binary-2_082_1	✅ cached	✅ cached	🚫 not supported
dmd-binary-2_083_1	✅ cached	✅ cached	🚫 not supported
dmd-binary-2_084_1	✅ cached	✅ cached	🚫 not supported
dmd-binary-2_085_1	✅ cached	✅ cached	🚫 not supported
dmd-binary-2_086_1	✅ cached	✅ cached	🚫 not supported
dmd-binary-2_087_1	✅ cached	✅ cached	🚫 not supported
dmd-binary-2_088_1	✅ cached	✅ cached	🚫 not supported
dmd-binary-2_089_1	✅ cached	✅ cached	🚫 not supported
dmd-binary-2_090_1	✅ cached	✅ cached	🚫 not supported
dmd-binary-2_098_0	✅ cached	✅ cached	🚫 not supported
dmd-bootstrap	✅ cached	✅ cached	🚫 not supported
dub	✅ cached	✅ cached	✅ cached
dub-1_30_0	✅ cached	✅ cached	✅ cached
ldc	✅ cached	🚧 known to fail (disabled)	🚧 known to fail (disabled)
ldc-1_30_0	✅ cached	🚧 known to fail (disabled)	🚧 known to fail (disabled)
ldc-binary-1_19_0	✅ cached	✅ cached	🚫 not supported
ldc-binary-1_25_0	✅ cached	✅ cached	✅ cached
ldc-binary-1_28_0	✅ cached	✅ cached	✅ cached
ldc-binary-1_32_1	✅ cached	✅ cached	✅ cached
ldc-binary-1_34_0	✅ cached	✅ cached	✅ cached
ldc-bootstrap	✅ cached	✅ cached	✅ cached