Build fails when using source from Releases page

### What happens?

Users who download the source code from the [Releases page](https://github.com/duckdb/pg_duckdb/releases) won't be able to follow the current build instructions

This is because [Makefile](https://github.com/duckdb/pg_duckdb/blob/main/Makefile#L115) assumes the source is in a git repository, which is not the case for release archives

### To Reproduce

1. Download and unzip source code from Releases page
2. Run `make`
```bash
root@14108219e845:/pg_duckdb-0.3.1# make
git submodule update --init --recursive
fatal: not a git repository (or any of the parent directories): .git
make: *** [Makefile:95: .git/modules/third_party/duckdb/HEAD] Error 128
```

### OS:

MacOS

### pg_duckdb Version (if built from source use commit hash):

0.3.1

### Postgres Version (if built from source use commit hash):

17.5

### Hardware:

_No response_

### Full Name:

Cheng Chen

### Affiliation:

Mooncake Labs

### What is the latest build you tested with? If possible, we recommend testing with the latest nightly build.

I have tested with a source build

### Did you include all relevant data sets for reproducing the issue?

Yes

### Did you include all code required to reproduce the issue?

- [x] Yes, I have

### Did you include all relevant configuration (e.g., CPU architecture, Linux distribution) to reproduce the issue?

- [x] Yes, I have

Start with the Makefile around the referenced line and reproduce the failure using the 0.3.1 source archive from the Releases page, then run make outside a Git checkout. Done means the documented release-archive build no longer fails because it assumes a Git repository and the source build completes successfully.

Written by the indexing model from the issue text.