Further additions to flake (#4489)

* Create a mkderivation wrapper lambda.

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* Removed execName

* Call new wrappers instead of duplicated mkDerivation.

* Removed finalAttrs

* Use more semi-colons

* Used clang and the build environment instead of the standard gcc.

* Replaced "git" with current nightly version.

* Automatically pass the required environment variables into the development shell instead of requiring them as part of cmake command.

* Change to an if statement because ! hurts the brain.

* Added perf to devshell package list.

* Added Nix formatter.

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* Collision with the ++ operator in the environment variable.

* Nix Format.