Does not compile due to bad git reference.

I tried to build this using just `cabal build all` and it failed with:
```
fatal: reference is not a tree: 5bcc67b3b2eb219a54b90c24920a72c9266190a2
```
this is due to the git reference in the `cabal.project` file:
```
location: https://github.com/input-output-hk/plutus-apps
tag: 5bcc67b3b2eb219a54b90c24920a72c9266190a2
```
because that commit is not on the `master` branch of the `plutus-apps` repo. I suspect that it references a PR commit and the commit hash changed when it was merged.

`db-sync` and `node` both have CI scripts that ensure the all git references are on the `master` branch. I encourage you do do the same in this repo.