flake: clean up devShells

[bloxx12]

Move both devShells into devshells.<...>,
packages should be used in shells, as the logical difference between nativeBuildInputs and buildInputs no longer applies.

can't really test since I don't have vivado installed, it should just work however.

ARTIQ Pull Request

Description of Changes

Related Issue

Type of Changes

	Type
✓	🔨 Refactoring

Steps (Choose relevant, delete irrelevant before submitting)

All Pull Requests

• Use correct spelling and grammar.
• Update RELEASE_NOTES.rst if there are noteworthy changes, especially if there are changes to existing APIs.
• Close/update issues.
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Code Changes

• Test your changes or have someone test them. Mention what was tested and how.