Remove --immutable-cache in yarn install command

Since Yarn 3.1.0, some cache items no longer have checksums, mostly
derived items like patched packages. Nixify skipped these, but that has
started causing `--immutable-cache` to complain about the missing
entries.

Instead, we will have to rely on the Nix sandbox for purity, so that
Yarn only does local operations (like patching) when to fill in these
missing cache entries.

src/tmpl/yarn-project.nix.in

@@ -77,7 +77,7 @@ let
       install -m 0600 ${lockfile} ./yarn.lock
       export yarn_global_folder="$TMP"
       export YARN_ENABLE_IMMUTABLE_INSTALLS=false
-      yarn --immutable-cache
+      yarn
     '';
 
     installPhase = ''
@@ -115,7 +115,7 @@ let
 @@ISOLATED_INTEGRATION@@
 
       # Run normal Yarn install to complete dependency installation.
-      yarn install --immutable --immutable-cache
+      yarn install --immutable
 
       runHook postConfigure
     '';