Fixup build-pivotal-file
Fixup build-pivotal-file

...so that it can be run from outside of the example folder

fixes
```
+ ops-manager-example-repo/build-pivotal-file 1.12.0-committed.20170817.121117-stemcell.3445 3445 /tmp/build/1a456e57/example-product-output
fatal: Not a git repository (or any of the parent directories): .git
```

Signed-off-by: Topher Lubaway <b16f5c2179c9bd81c63bb4363d889b349d49366b@pivotal.io>

pivotal-cf-experimental/ops-manager-example,pivotal-cf-experimental/ops-manager-example,pivotal-cf-experimental/ops-manager-example,pivotal-cf-experimental/ops-manager-example