test: build the fallback message with format!, matching the commit before it

`4057bee` claimed the report prints

  SYLPHEED_DISC   PRESENT via the HARDCODED fallback, NOT $SYLPHEED_DISC

and it did not. The string was built by `"...(not ${env})".replace("${env}",
env)` -- a placeholder substituted at runtime, which printed
`(not SYLPHEED_DISC)` and dropped the `$`. I had cleaned this up before
committing, but the edit silently no-op'd (rustfmt had already rewrapped the
line, so the pattern no longer matched) and the commit went ahead with the
message quoting output the code does not produce.

Now a plain `format!`, which is what it should have been: shorter, no
placeholder to keep in sync, and the `$` makes it read as the env var it is.
The quoted output in `4057bee` is accurate as of this commit.

Not squashed into it because force-push is denied on this remote, and a
visible correction is worth more than a rewritten history anyway.

Verified: `cargo fmt --all -- --check` clean, both tests pass, output matches
the text above exactly.

Refs #16

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

crates/sylpheed-formats/tests/corpus_report.rs

@@ -58,7 +58,7 @@ fn resolve(
         // so the suites run because of this machine's directory layout — a
         // property invisible in the command and in the output.
         return (
-            "PRESENT via the HARDCODED fallback (not ${env})".replace("${env}", env),
+            format!("PRESENT via the HARDCODED fallback, NOT ${env}"),
             Some(p),
         );
     }