ci: Configure release-plz to run semver checks on libm

Author: tgross35

.release-plz.toml

@@ -1,7 +1,10 @@
-[workspace]
+[[package]]
+name = "compiler_builtins"
 semver_check = false
-
 # As part of the release process, we delete `libm/Cargo.toml`. Since
 # this is only run in CI, we shouldn't need to worry about it.
 allow_dirty = true
 publish_allow_dirty = true
+
+[[package]]
+name = "libm"