opentitan dev env improvements

[nbdd0121]

• Bazel 7 compatibility fixed (by setting TMPDIR to appease Bazel)
• Implement a overlay-based FHS env. Compared to the nixpkgs FHS envs, this have the advantage of preserving existing contents in /usr/lib and /usr/bin on already FHS distros. It also avoids broken /etc/localtime issue that @rswarbrick is seeing.
• Fix opentitan.nix FHSenv messes up PS1 (invokes non-interactive bash?) #55
  • This is caused by nix: develop: use SHELL from rc script NixOS/nix#8043 which doesn't allow us to read $SHELL anymore:
  • The dev shell now needs to be launched with nix run lowrisc-nix#opentitan-dev-env instead of nix develop lowrisc-nix#opentitan.
  • EDIT: I have changed the approach to use /proc/$PPID/exe to detect parent shell.

[hcallahan-lowrisc]

Thanks @nbdd0121 ! This seems like a cool pragmatic approach to make the impure development environment work better cross-distro within our requirements. It sounds like you already have some test coverage of this working with @rswarbrick on Debian.
My initial reaction was that I don’t love the idea of maintaining this extra infrastructure, but maybe it’s worth it. Let’s run with this for a while and see how it goes.
LGTM