Upgrade to Yarn v3

src/tmpl/yarn-project.nix.in

@@ -75,6 +75,8 @@ let
 
       echo '{ "dependencies": { "${pname}": "${version}" } }' > package.json
       install -m 0600 ${lockfile} ./yarn.lock
+      export yarn_global_folder="$TMP"
+      export YARN_ENABLE_IMMUTABLE_INSTALLS=false
       yarn --immutable-cache
     '';
 
@@ -107,6 +109,9 @@ let
       source ${mkCacheBuilderForDrvs (lib.attrValues cacheDrvs)}
       popd > /dev/null
 
+      # Yarn may need a writable home directory.
+      export yarn_global_folder="$TMP"
+
 @@ISOLATED_INTEGRATION@@
 
       # Run normal Yarn install to complete dependency installation.