vscode-utils: Set the sourceRoot attribute on vscode extensions

Some extensions contain directories other than `extension` (e.g.,
`package`, if the extension has a digital signature). The unpacker phase
fails if a .vsix file contains more than one directory at the archive's
root and the `sourceRoot` attribute is unset.

(cherry picked from commit 8f384467cc)

pkgs/applications/editors/vscode/extensions/vscode-utils.nix

@@ -32,6 +32,10 @@ let
 
     inherit configurePhase buildPhase dontPatchELF dontStrip;
 
+    # Some .vsix files contain other directories (e.g., `package`) that we don't use.
+    # If other directories are present but `sourceRoot` is unset, the unpacker phase fails.
+    sourceRoot = "extension";
+
     installPrefix = "share/vscode/extensions/${vscodeExtUniqueId}";
 
     nativeBuildInputs = [ unzip ] ++ nativeBuildInputs;