nix: Add nightly build job with cachix (#27014)

I'll be using this to `nix run github:zed-industries/zed/nightly` and
get an up-to-date and cached nightly build.

It'll also serve as a way to warn me when the nix build is broken,
rather than having to wait for users to report it.

Eventually and depending on the build time of the nix builds, we may
want to consider putting a nix build in CI (#17458) to prevent
breakages, but for now a best-effort nightly build that doesn't block
the job if it fails is a good start.

Resolve #19937

Release Notes:

- N/A

nix/shell.nix

@@ -49,6 +49,8 @@ mkShell' {
     (removeAttrs baseEnvs [
       "LK_CUSTOM_WEBRTC" # download the staticlib during the build as usual
       "ZED_UPDATE_EXPLANATION" # allow auto-updates
+      "CARGO_PROFILE" # let you specify the profile
+      "TARGET_DIR"
     ])
     // {
       # note: different than `$FONTCONFIG_FILE` in `build.nix` – this refers to relative paths