Fix missing digest for m.jar (pantsbuild#8447)

### Problem

```
[ 27/207] Rsc-ing 302 mixed sources in 1 target ('<some target>).
rsc(finagle/finagle-core/src/main:main) failed: 
ClasspathEntry ClasspathEntry(path='<some path>/source/.pants.d/compile/rsc/c1e3836b60e5/finagle.finagle-core.src.main.main/current/rsc/m.jar', directory_digest=None) didn't have a Digest, so won't be present for hermetic 
```

The issue is that when we are setting the directory like this
```
ctx.rsc_jar_file = ClasspathEntry(ctx.rsc_jar_file.path, res.output_directory_digest)
```
it creates a new object and assign it to `ctx.rsc_jar_file`, whereas somewhere else in the code still points to the `ClasspathEntry` that `ctx.rsc_jar_file` used to point to.

### Solution

Do not create an object, but change its attribute.

### Result

RSC does not choke on this anymore.

Author: wisechengyi

src/python/pants/backend/jvm/tasks/jvm_compile/rsc/rsc_compile.py

@@ -674,15 +674,13 @@ def _runtool_hermetic(self, main, tool_name, distribution, input_digest, ctx):
     # TODO: parse the output of -Xprint:timings for rsc and write it to self._record_target_stats()!
 
     res.output_directory_digest.dump(ctx.rsc_jar_file.path)
-
-    ctx.rsc_jar_file = ClasspathEntry(ctx.rsc_jar_file.path, res.output_directory_digest)
-
     self.context._scheduler.materialize_directories((
       DirectoryToMaterialize(
         # NB the first element here is the root to materialize into, not the dir to snapshot
         get_buildroot(),
         res.output_directory_digest),
     ))
+    ctx.rsc_jar_file.hydrate_missing_directory_digest(res.output_directory_digest)
 
     return res