pass through $SHELL

[figsoda]

https://github.com/NixOS/nix/commit/27be54ca533933db8c3e0cde4b213abf10dd5237

this fixes the issue where my prompt (fish + starship) gets jumbled up if I riff shell in a riff shell

before:

~/riff on  main 
❯ result/bin/riff shell
✓ 🦀 rust: cargo, openssl, pkg-config, rustc, rustfmt

~/riff on  main via  impure (riff-shell) 
❯ result/bin/riff shell
✓ 🦀 rust: cargo, openssl, pkg-config, rustc, rustfmt

\[\]~/riff\[\] on \[\] \[\]\[\]main\[\] via \[\] \[\]\[\]impure\[\]\[\] (\[\]\[\]riff-shell\[\]\[\])\[\] 
\[\]❯\[\] 

after:

~/riff on  shell 
❯ result/bin/riff shell
✓ 🦀 rust: cargo, openssl, pkg-config, rustc, rustfmt

~/riff on  shell via  impure (riff-shell) 
❯ result/bin/riff shell
✓ 🦀 rust: cargo, openssl, pkg-config, rustc, rustfmt

~/riff on  shell via  impure (riff-shell) 
❯