build-unit-test-docker: fix clang version argument

The latest version of bash seems to not pass additional arguments
when called with `-c` unless those arguments are after the `--`.

This caused the build-unit-test-docker to not install the latest
version of clang, but instead the release of clang that was present
in the Ubuntu repository.

Add "19" after `--` so that the latest versions of clang tools are
installed.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I894b77b2a03e9125df50a297c9898b3456846cdc
diff --git a/scripts/build-unit-test-docker b/scripts/build-unit-test-docker
index 49c6fb4..afcdc8c 100755
--- a/scripts/build-unit-test-docker
+++ b/scripts/build-unit-test-docker
@@ -903,7 +903,7 @@
     update-alternatives --install /usr/bin/cpp cpp /usr/bin/cpp-14 14
 
 # Set up LLVM apt repository.
-RUN bash -c "$(wget -O - https://apt.llvm.org/llvm.sh)" 19
+RUN bash -c "$(wget -O - https://apt.llvm.org/llvm.sh)" -- 19
 
 # Install extra clang tools
 RUN apt-get install -y \