Bump cross-spawn from 5.1.0 to 6.0.6 in /packages/generator-volto

Bumps [cross-spawn](https://github.com/moxystudio/node-cross-spawn) from 5.1.0 to 6.0.6.
- [Changelog](https://github.com/moxystudio/node-cross-spawn/blob/v6.0.6/CHANGELOG.md)
- [Commits](moxystudio/node-cross-spawn@5.1.0...v6.0.6)

---
updated-dependencies:
- dependency-name: cross-spawn
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

packages/generator-volto/package.json

@@ -68,7 +68,7 @@
     "ansi-escapes": "2.0.0",
     "chalk": "^2.1.0",
     "commander": "2.9.0",
-    "cross-spawn": "5.1.0",
+    "cross-spawn": "6.0.6",
     "execa": "0.6.3",
     "fs-extra": "3.0.0",
     "gitly": "2.0.3",