Adjust rpath in cabal-cache artifacts

Problem:
```
[ 9 of 29] Compiling App.Options.Lens ( src/App/Options/Lens.hs, /Users/david/src/arbor/mayhem/banner-commander/dist-newstyle/build/x86_64-osx/ghc-8.6.5/banner-commander-0.0.1/build/App/Options/Lens.o ) [Control.Lens changed]
: can't load .so/.DLL for: /Users/david/.cabal/store/ghc-8.6.5/lib/libHSrndm-shffl-0.0.4-63f5dfd2-ghc8.6.5.dylib (dlopen(/Users/david/.cabal/store/ghc-8.6.5/lib/libHSrndm-shffl-0.0.4-63f5dfd2-ghc8.6.5.dylib, 5): Library not loaded: /usr/local/opt/ghc/lib/ghc-8.6.5/base-4.12.0.0/libHSbase-4.12.0.0-ghc8.6.5.dylib
Referenced from: /Users/david/.cabal/store/ghc-8.6.5/lib/libHSrndm-shffl-0.0.4-63f5dfd2-ghc8.6.5.dylib
Reason: image not found)
```

sometimes an .so is pushed that has a full path to a base library.

```
otool -L lib/libHSrndm-shffl-0.0.4-63f5dfd2-ghc8.6.5.dylib
lib/libHSrndm-shffl-0.0.4-63f5dfd2-ghc8.6.5.dylib:
@rpath/libHSrndm-shffl-0.0.4-63f5dfd2-ghc8.6.5.dylib (compatibility version 0.0.0, current version 0.0.0)
@rpath/libHSrndm-1.1-46d8c4d3-ghc8.6.5.dylib (compatibility version 0.0.0, current version 0.0.0)
/usr/local/opt/ghc/lib/ghc-8.6.5/base-4.12.0.0/libHSbase-4.12.0.0-ghc8.6.5.dylib (compatibility version 0.0.0, current version 0.0.0)
/usr/local/opt/ghc/lib/ghc-8.6.5/integer-gmp-1.0.2.0/libHSinteger-gmp-1.0.2.0-ghc8.6.5.dylib (compatibility version 0.0.0, current version 0.0.0)
/usr/local/opt/ghc/lib/ghc-8.6.5/ghc-prim-0.5.3/libHSghc-prim-0.5.3-ghc8.6.5.dylib (compatibility version 0.0.0, current version 0.0.0)
/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1252.250.1)
```

They should all be relative to `@rpath`:

```
ghc-8.6.5/lib/libHSBln-0.2.4-a71178c6-ghc8.6.5.dylib:
@rpath/libHSBln-0.2.4-a71178c6-ghc8.6.5.dylib (compatibility version 0.0.0, current version 0.0.0)
@rpath/libHSbase-4.12.0.0-ghc8.6.5.dylib (compatibility version 0.0.0, current version 0.0.0)
@rpath/libHSinteger-gmp-1.0.2.0-ghc8.6.5.dylib (compatibility version 0.0.0, current version 0.0.0)
@rpath/libHSghc-prim-0.5.3-ghc8.6.5.dylib (compatibility version 0.0.0, current version 0.0.0)
```