[Build] don't install rsync if vcache is disabled

[k-v1]

Why I did it

We don't need to install rsync in every docker container if vcache is disabled.

How I did it

Install rsync in pre_run_buildinfo script only if vcache is enabled.

How to verify it

[xumia]

LGTM, the build failure may be caused by not running the command "apt-get update". I sent another PR to fix it, #14439. @k-v1, you also cherry-pick the commit in your PR, or rerun the test after the PR merged.

Step 14/77 : RUN apt-get install -y gnupg
 ---> Running in 803670deee75
Reading package lists...
Building dependency tree...
Reading state information...
Package gnupg is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'gnupg' has no installation candidate
The command '/bin/sh -c apt-get install -y gnupg' returned a non-zero code: 100

[xumia]

/azp run Azure.sonic-buildimage

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).