This worked with pacman 5, but fails with pacman 6

* Use the following DB in /etc/pacman.conf:

```
[mingw64]
Server = https://mirror.msys2.org/mingw/mingw64/
```

* Run `pacman -Syy`

Expected: It works

Actual: error: mingw64: missing required signature
error: failed to synchronize all databases (invalid or corrupted database (PGP signature))

----

The problem is that the mirror is a https://github.com/etix/mirrorbits instance
which (1) redirects all URLs to a mirror and (2) de-refs symlinks before doing
so. This means there will be redirects with a different filename for each DB
related file.

The first request gives:

$ curl -I https://mirror.msys2.org/mingw/x86_64/mingw64.db
HTTP/2 302
cache-control: private, no-cache
content-type: text/html; charset=utf-8
date: Wed, 16 Jun 2021 20:21:05 GMT
link: <https://repo.msys2.org/mingw/x86_64/mingw64.db.tar.gz>; rel=duplicate; pri=1; geo=de
location: https://mirror.yandex.ru/mirrors/msys2/mingw/x86_64/mingw64.db.tar.gz
server: Mirrorbits/v0.5.1-31-geeea0e0-dirty

This results in pacman doing the following:

debug: mingw64.db: url is https://mirror.msys2.org/mingw/x86_64/mingw64.db
debug: mingw64.db: maxsize 134217728
debug: mingw64.db: opened tempfile for download: /var/lib/pacman/sync/mingw64.db.part (wb)
debug: mingw64.db: curl returned result 0 from transfer
debug: mingw64.db: response code 200

Then it tries to fetch the signature file:

debug: mingw64.db.tar.gz.sig: url is https://repo.msys2.org/mingw/x86_64/mingw64.db.tar.gz.sig
debug: mingw64.db.tar.gz.sig: maxsize 16384
debug: mingw64.db.tar.gz.sig: opened tempfile for download: /var/lib/pacman/sync/mingw64.db.tar.gz.sig.part (wb)
debug: mingw64.db.tar.gz.sig: curl returned result 0 from transfer
debug: mingw64.db.tar.gz.sig: response code 200

Then it fails to find the signature file on disk and fails:

debug: "/var/lib/pacman/sync/mingw64.db.sig" is not readable: No such file or directory
debug: sig path /var/lib/pacman/sync/mingw64.db.sig could not be opened
debug: got error 43 at _alpm_gpgme_checksig (../lib/libalpm/signing.c: 593) : missing PGP signature
debug: missing required signature
error: mingw64: missing required signature
debug: failed to validate db: mingw64

The filesystem contains a /var/lib/pacman/sync/mingw64.db.tar.gz.sig though

I know that this is a bit of a weird edge case, so if this is out of scope I
understand. I'll try to teach(hack) mirrorbits to not de-reference symlinks
instead.