Upgrade rust/crates/nix to 0.21.0

Test: make
Change-Id: Ic8422bfdc4afc11c6c310e67999ec360d92d843d
diff --git a/test/common/mod.rs b/test/common/mod.rs
index a871b47..8a79d6a 100644
--- a/test/common/mod.rs
+++ b/test/common/mod.rs
@@ -114,8 +114,8 @@
                 let mut version = Version::parse(fixed_release).unwrap();
 
                 //Keep only numeric parts
-                version.pre.clear();
-                version.build.clear();
+                version.pre = semver::Prerelease::EMPTY;
+                version.build = semver::BuildMetadata::EMPTY;
 
                 if !version_requirement.matches(&version) {
                     skip!("Skip {} because kernel version `{}` doesn't match the requirement `{}`",