pgplot 5.2.2: Add mirror

```
❯❯❯ brew fetch pgplot --build-from-source
==> Fetching pgplot from homebrew/homebrew-x11
==> Downloading ftp://ftp.astro.caltech.edu/pub/pgplot/pgplot522.tar.gz

curl: (28) Timeout was reached
Error: Failed to download resource "pgplot"
Download failed: ftp://ftp.astro.caltech.edu/pub/pgplot/pgplot522.tar.gz
```