precompilepkgs: simplify custom config printing if only one (#53805)
Currently it's a bit excessive in the `Pkg.test` precompile job

This PR
```
Precompiling project for configuration --code-coverage=none --color=yes --check-bounds=yes --warn-overwrite=yes --depwarn=yes --inline=yes --startup-file=no --track-allocation=none...
354.9 ms ✓ RFFT
1 dependency successfully precompiled in 1 seconds. 38 already precompiled.
```
Pkg could also just set the non-default flags to minimize the list.