Exclude tests from MSRV testing recipe

In practice, dev-deps often have much more lenient MSRV policy than
then crate under test. Using `build` rather than `test` works around
that.

Author: matklad

src/necessities.md

@@ -146,5 +146,5 @@ pinned to minimal versions:
 
 ```bash
 $ cp ci/Cargo.lock.min ./Cargo.lock
-$ cargo +$MSRV test
+$ cargo +$MSRV build
 ```