build: pass --abbrev to git rather than relying on global config

For computing the zig version number, pass --abbrev=9 rather than
requiring the user to set their git configuration in order to make zig
versions match the standard.

ci/x86_64-windows-debug.ps1

@@ -23,7 +23,6 @@ function CheckLastExitCode {
 
 # Make the `zig version` number consistent.
 # This will affect the `zig build` command below which uses `git describe`.
-git config core.abbrev 9
 git fetch --tags
 
 if ((git rev-parse --is-shallow-repository) -eq "true") {