patch starship until starship/starship#2807 lands

starship does not have built-in support for xonsh yet, but there are
multiple open efforts to add it, including a PR I opened recently:

    starship/starship#2807

Until that PR lands, I'm switching to using my patched version.

(I was already using a patched version on my mac, which I built locally.
Better to override it in the nix config so I automatically get it on
other machines too.)

config/xonsh/rc.xsh

@@ -36,4 +36,4 @@ aliases['ff'] = lambda: $['$HOME/.nix-profile/bin/ff']
 
 execx($(colorscheme restore-colors))
 
-execx($(/Users/jeremy/src/github.com/starship/starship/target/release/starship init xonsh))
+execx($(starship init xonsh))

flake.nix

@@ -210,7 +210,15 @@
         ripgrep               # Text search. (Phenomenal grep replacement.)
         sd                    # Text find-and-replace. (Decent sed replacement.)
         stack                 # Build haskell projects.
-        starship              # Nice command prompt.
+        (starship.overrideAttrs # Nice command prompt.
+          (oldAttrs: {
+            src = fetchFromGitHub {
+              owner = "jeremyschlatter";
+              repo = "starship";
+              rev = "xonsh";
+              sha256 = "sha256-CuOpIb9vGHIUY19IH3cHi7gnXK8v+GQtIMTmjJ0UjBo=";
+            };
+          }))
         tldr                  # Show usage examples for common CLI programs.
         unzip                 # Open .zip files.
         watch                 # Run a command repeatedly.