fix(cli): Provide next steps for bad -Z flag

In rust-lang#5546, they suggested we have a list of them but that would be hard
to keep up and works well enough to just always mention it.

Fixes rust-lang#5546

src/cargo/core/features.rs

@@ -1097,7 +1097,10 @@ impl CliUnstable {
             "script" => self.script = parse_empty(k, v)?,
             "target-applies-to-host" => self.target_applies_to_host = parse_empty(k, v)?,
             "unstable-options" => self.unstable_options = parse_empty(k, v)?,
-            _ => bail!("unknown `-Z` flag specified: {}", k),
+            _ => bail!("\
+            unknown `-Z` flag specified: {k}\n\n\
+            For available unstable features, see https://doc.rust-lang.org/nightly/cargo/reference/unstable.html\n\
+            If you intended to use an unstable rustc feature, try setting `RUSTFLAGS=\"-Z{k}\"`"),
         }
 
         Ok(())

tests/testsuite/cargo_features.rs

@@ -594,7 +594,13 @@ fn z_flags_rejected() {
     p.cargo("check -Zarg")
         .masquerade_as_nightly_cargo(&["test-dummy-unstable"])
         .with_status(101)
-        .with_stderr("error: unknown `-Z` flag specified: arg")
+        .with_stderr(
+            r#"error: unknown `-Z` flag specified: arg
+
+For available unstable features, see https://doc.rust-lang.org/nightly/cargo/reference/unstable.html
+If you intended to use an unstable rustc feature, try setting `RUSTFLAGS="-Zarg"`
+"#,
+        )
         .run();
 
     p.cargo("check -Zprint-im-a-teapot")