ci: Fix rust test compilation (linkerd#12366)

We expect to run test compilation independently of test execution, but a change
to test runner has dropped support for --message-format=json (in favor of
--message-format=libtest-json, etc).

This commit updates the test runner invocation to stop formatting output in
these steps.

Signed-off-by: Mark S <the@wondersmith.dev>

justfile

@@ -66,8 +66,7 @@ rs-doc *flags:
 rs-test-build:
     {{ _cargo-test }} --no-run --frozen \
         --workspace --exclude=linkerd-policy-test \
-        {{ _features }} \
-        {{ _fmt }}
+        {{ _features }}
 
 # Run Rust unit tests
 rs-test *flags:
@@ -144,7 +143,7 @@ policy-test-run *flags:
 
 # Build the policy tests without running them.
 policy-test-build:
-    cd policy-test && {{ _cargo-test }} --no-run {{ _fmt }}
+    cd policy-test && {{ _cargo-test }} --no-run
 
 # Delete all test namespaces and remove linkerd from the cluster.
 policy-test-cleanup: