Fix dependency checksum error during initial setup

[icebound777]

The provided URL for mips-gcc-egcs-2.91.66-linux.tar.gz within ./shell.nix points to the latest release of that repository.

However, that repository recently saw new releases, so a different file now gets downloaded and the provided checksum for that file no longer matches.

error: hash mismatch in file downloaded from 'https://github.com/decompals/mips-gcc-egcs-2.91.66/releases/latest/download/mips-gcc-egcs-2.91.66-linux.tar.gz':
         specified: sha256:03v1ci7j0hi53z639rwj60xwz0zzi82a9azi0yiw818r754faql0
         got:       sha256:1fdvlah66r26c4fvy4j97f2bqaair19bbbg4z3x0qz16iphhzmhx

This PR pins the release of that dependency to its version 0.2, which matches the provided sha256 checksum.

As an alternative to this PR, if an update to a newer version of the dependency is being considered, I recommend both updating the sha256 to the current up-to-date version of 0.4, as well as pinning the dependency to said version.

For reference, see https://github.com/decompals/mips-gcc-egcs-2.91.66/releases

[bates64]

Thanks!