jjb: Clone linux kernel with `--reference-if-able`

jjb: Clone linux kernel with `--reference-if-able`

lttng-modules_PARAM_crossbuild jobs for riscv64 were intermittently
failing when cloning `linux-all.git` with messages similar to the
following:

```
11:53:56 + git clone --depth=1 -b v4.10.17 --reference /home/jenkins/gitcache/linux-stable.git/ git://git-mirror.internal.efficios.com/git/linux-all.git /home/jenkins/workspace/lttng-modules_PARAM_crossbuild/cross_arch/riscv64/platforms/deb12-amd64/src/linux
11:53:56 Cloning into '/home/jenkins/workspace/lttng-modules_PARAM_crossbuild/cross_arch/riscv64/platforms/deb12-amd64/src/linux'...
11:53:56 remote: fatal: packfile ./objects/pack/.tmp-479230-pack-f8bc46fc38bfd97fc2a516d9816a64fccfbfd3b2.pack cannot be accessed
11:53:56 remote: aborting due to possible repository corruption on the remote side.
11:53:56 fatal: early EOF
11:53:56 fatal: fetch-pack: invalid index-pack output
```

Testing using `--reference-if-able` instead of `--reference` seems to
resolve the cloning issue.

Change-Id: I49a03190604c00095436f3098dd8ac95706094e0
Signed-off-by: Kienan Stewart <kstewart@efficios.com>