fix: pre-commit errors due to polluted git directory

[vlaci]

After #396, reuse hook no longer works, as it considers untracked files added by pre-commit. Before the above PR, the git repository was in $HOME/src, so files added in $HOME didn't affect the outcome of the hooks themselves.

This change re-adds the src subdirectory.

A failing build output:

 $ nix build --no-link -L .\#checks.x86_64-linux.pre-commit
 Running phase: unpackPhase
 unpacking source archive /nix/store/g1p6g2ylflj4fnzdwlc368737kxk0p9f-source
 source root is source
 Running phase: patchPhase
 Running phase: updateAutotoolsGnuConfigScriptsPhase
 Running phase: configurePhase
 no configure script, doing nothing
 Running phase: buildPhase
 Running: $ pre-commit run --all-files
 reuse....................................................................Failed
 - hook id: reuse
 - exit code: 1
 # MISSING COPYRIGHT AND LICENSING INFORMATION
 The following files have no copyright and licensing information:
 * .cache/pre-commit/README
 * .cache/pre-commit/db.db
 * .gitconfig
 # SUMMARY
 * Bad licenses: 0
 * Deprecated licenses: 0
 * Licenses without file extension: 0
 * Missing licenses: 0
 * Unused licenses: 0
 * Used licenses: MIT
 * Read errors: 0
 * Files with copyright information: 16 / 19
 * Files with license information: 16 / 19
 Unfortunately, your project is not compliant with version 3.3 of the REUSE Specification :-(
 # RECOMMENDATIONS
 * Fix missing copyright/licensing information: For one or more files, the tool
   cannot find copyright and/or licensing information. You typically do this by
   adding 'SPDX-FileCopyrightText' and 'SPDX-License-Identifier' tags to each
   file. The tutorial explains additional ways to do this:
   <https://reuse.software/tutorial/>

[sandydoo]

@vlaci, thanks for bringing this up.

Before the linked PR, we would also copy the source files into the src directory. If we cd into an empty src directory, then the hooks will have nothing to run against.

I've merged #551, which sets HOME to a temporary directory to avoid polluting the working directory.