graphite-cli: fix darwin build

The 1.8.6 bump (#527044) broke the package on darwin in two independent
ways, unnoticed because the package is unfree and therefore never built
by Hydra or ofborg:

- postInstall generates shell completions by running $out/bin/gt, but
  gt tries to create ~/.config/graphite/aliases on startup and exits 1
  with no output when HOME is not writable (as in nix builds, where
  HOME=/homeless-shelter). installShellCompletion then fails the build
  because the generated completion files are zero-size. Give the build
  a throwaway writable HOME. The Linux build is not affected because
  completions are generated through the buildFHSEnv wrapper, where /
  is a writable tmpfs.

- dontFixup was only set on Linux, so on darwin fixupPhase ran strip -S
  on the binary. As the comment in this file already explains, the
  vercel/pkg virtual filesystem appended to the binary must not be
  modified; the stripped binary fails at runtime with "Pkg: Error
  reading from file." Skip fixup on all platforms. On Linux this is a
  no-op: the derivation is unchanged.

Assisted-by: Devin (Ultra)
Co-Authored-By: Rahul Chalamala <22563365+rchalamala@users.noreply.github.com>

pkgs/by-name/gr/graphite-cli/package.nix

@@ -76,9 +76,10 @@ let
 
     dontConfigure = true;
     dontBuild = true;
-    # On Linux the binary is wrapped with buildFHSEnv; completions are
-    # generated there. Here we only need to skip fixup to avoid patchelf/strip.
-    dontFixup = stdenv.hostPlatform.isLinux;
+    # Skip fixup on all platforms: strip discards the vercel/pkg virtual
+    # filesystem appended to the binary (see the comment below), leaving a
+    # binary that fails at runtime with "Pkg: Error reading from file."
+    dontFixup = true;
 
     installPhase = ''
       runHook preInstall
@@ -86,7 +87,13 @@ let
       runHook postInstall
     '';
 
-    postInstall = lib.optionalString stdenv.hostPlatform.isDarwin shellCompletions;
+    # gt tries to create ~/.config/graphite/aliases on startup and exits 1
+    # with no output when HOME is not writable, which would leave the
+    # completion files empty.
+    postInstall = lib.optionalString stdenv.hostPlatform.isDarwin ''
+      export HOME=$(mktemp -d)
+      ${shellCompletions}
+    '';
   };
 in
 # The binary is built with vercel/pkg, which appends a virtual filesystem to