build: Set `resolver = "2"` at the workspace's Cargo.toml

This is the default resolver for the 2021 edition, but needs to be set
manually in virtual workspaces.

https://doc.rust-lang.org/edition-guide/rust-2021/default-cargo-resolver.html#detail

Cargo.toml

@@ -1,2 +1,3 @@
 [workspace]
+resolver = "2"
 members = ["xtask", "{{project-name}}", "{{project-name}}-common"]