ci: Fix the fuchsia tests

The target triple name was changed in rust-lang/rust#106429

While `rustc` known the old spelling of the triple, `rustup` does not,
so the toolchain download was not working.

Signed-off-by: Joe Richey <joerichey@google.com>

.github/workflows/tests.yml

@@ -268,7 +268,7 @@ jobs:
     strategy:
       matrix:
         target: [
-          x86_64-fuchsia,
+          x86_64-unknown-fuchsia,
           x86_64-unknown-redox,
           x86_64-fortanix-unknown-sgx,
         ]