Set allow_dirty to release-plz

This should resolve the issue in
<rust-lang#643 (comment)>

Author: tgross35

.release-plz.toml

@@ -1,3 +1,8 @@
 [workspace]
 changelog_update = false
 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