RUNTIME_PREFIX references in gitconfig variable paths

One thing I wanted to do was provide a default ca-bundle.crt file along with my local build of Git. I need my installation to be relocatable and I'm using RUNTIME_PREFIX with Git 2.18.0 (on GNU/Linux).

I can provide a system gitconfig file with a setting for http.sslCAInfo but the problem is I can't create a relocatable path here so I don't know how to set it:

  $ cat $prefix/etc/gitconfig
  [http]
      sslCAInfo = <prefix>/etc/ca-bundle.crt

What do I use for <prefix> above since I want it to be relocatable? Basically I want this to be in the same directory as the relocatable sysconfdir (I don't actually care much but that seems like a good place).

Is there some way to create a reference to a path relative to the installation directory?

For example "~" is accepted as the users $HOME path; is there some syntax which refers to the Git installation directory?

If not this seems like something that would be very useful.

I can use a wrapper script and set GIT_SSL_CAINFO, but that will also override any user's setting of http.sslCAInfo in their local gitconfig which I don't really want.