[nix] hack to make libgit2-sys builds more reliable

Signed-off-by: Lucy Menon <168595099+syntactically@users.noreply.github.com>

Author: syntactically

flake.nix

@@ -152,14 +152,28 @@
             makeClauses = lib.strings.concatStringsSep "\n"
               (lib.mapAttrsToList makeClause deps);
           in ''
-            manifest=$(''${base}/bin/cargo locate-project --message-format plain --workspace)
+            base_cargo() {
+              PATH="$base/bin:$PATH" "$base/bin/cargo" "$@"
+            }
+
+            manifest=$(base_cargo locate-project --message-format plain --workspace)
             case "$manifest" in
               ${matchClauses}
             esac
             if [ -f ''${root}/flake.nix ]; then
               sed -i '/# vendor dependency configuration generated by nix/{N;d;}' $root/.git/info/exclude
               ${makeClauses}
             fi
+
+            # libgit2-sys copies a vendored git2 into the target/
+            # directory somewhere. In certain, rare, cases,
+            # libgit2-sys is rebuilt in the same incremental dep
+            # directory as it was before, and then this copy fails,
+            # because the files, copied from the nix store, already
+            # exist and do not have w permission. Hack around this
+            # issue by making any existing libgit2-sys vendored git2
+            # files writable before a build can be run
+            find "$(base_cargo metadata --format-version 1 | jq -r '.target_directory')" -path '*/build/libgit2-sys-*/out/include' -print0 | xargs -r -0 chmod u+w -R
           '';
 
           # Hyperlight scripts use cargo in a bunch of ways that don't