build-unit-test-docker: cppcheck update to latest

The 2.9 release seems to be the latest and contains the previous
commit we had referenced off the mainline.  Switch to use the
latest release tag.

```
$ git tag --contains 27578e9c4c1f90c62b6938867735a054082e178e
2.9
```

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I375b8cd1ef4c9f24bea3dbeac4744d31e4a2b63e
diff --git a/scripts/build-unit-test-docker b/scripts/build-unit-test-docker
index c875c4a..739f74a 100755
--- a/scripts/build-unit-test-docker
+++ b/scripts/build-unit-test-docker
@@ -94,7 +94,7 @@
         build_steps=[f"cp -a include/cereal/ {prefix}/include/"],
     ),
     "danmar/cppcheck": PackageDef(
-        rev="27578e9c4c1f90c62b6938867735a054082e178e",
+        rev="2.9",
         build_type="cmake",
     ),
     "CLIUtils/CLI11": PackageDef(