[DATALAD RUNCMD] Use our mirror instead of snapshot

Although snapshots might be more "official" and thus more "reliably present"
than our own server,  snapshots. has all kinds of throttling settings which
delay download or even cause it to fail:
datalad/datalad-installer#154

although datalad-installer should retry now (after it gets upgraded within
appveyor setup see datalad#7380) if
size changes, I think it would still be more robust to just get it from our
server.

I also made a note in
datalad/datalad-installer#160
so may be we gain possibility to specify multiple URLs thus to robustify.

=== Do not change lines below ===
{
 "chain": [],
 "cmd": "sed -i -e s,http://snapshot.debian.org/archive/debian/20210906T204127Z/pool/main/g/git-annex/,https://datasets.datalad.org/datalad/packages/neurodebian/,g .appveyor.yml",
 "exit": 0,
 "extra_inputs": [],
 "inputs": [],
 "outputs": [
  ".appveyor.yml"
 ],
 "pwd": "."
}
^^^ Do not change lines above ^^^

.appveyor.yml

@@ -79,7 +79,7 @@ environment:
       INSTALL_SYSPKGS: python3-virtualenv
       CODECOV_BINARY: https://uploader.codecov.io/latest/linux/codecov
       # system git-annex is way too old, use better one
-      INSTALL_GITANNEX: git-annex -m deb-url --url http://snapshot.debian.org/archive/debian/20210906T204127Z/pool/main/g/git-annex/git-annex_8.20210903-1_amd64.deb
+      INSTALL_GITANNEX: git-annex -m deb-url --url https://datasets.datalad.org/datalad/packages/neurodebian/git-annex_8.20210903-1_amd64.deb
     # Windows core tests
     - ID: WinP39core
       # ~35 min
@@ -119,7 +119,7 @@ environment:
       INSTALL_SYSPKGS: python3-virtualenv
       CODECOV_BINARY: https://uploader.codecov.io/latest/linux/codecov
       # system git-annex is way too old, use better one
-      INSTALL_GITANNEX: git-annex -m deb-url --url http://snapshot.debian.org/archive/debian/20210906T204127Z/pool/main/g/git-annex/git-annex_8.20210903-1_amd64.deb
+      INSTALL_GITANNEX: git-annex -m deb-url --url https://datasets.datalad.org/datalad/packages/neurodebian/git-annex_8.20210903-1_amd64.deb
     - ID: WinP39a1
       # ~40min
       DTS: >
@@ -182,7 +182,7 @@ environment:
       INSTALL_SYSPKGS: python3-virtualenv
       CODECOV_BINARY: https://uploader.codecov.io/latest/linux/codecov
       # system git-annex is way too old, use better one
-      INSTALL_GITANNEX: git-annex -m deb-url --url http://snapshot.debian.org/archive/debian/20210906T204127Z/pool/main/g/git-annex/git-annex_8.20210903-1_amd64.deb
+      INSTALL_GITANNEX: git-annex -m deb-url --url https://datasets.datalad.org/datalad/packages/neurodebian/git-annex_8.20210903-1_amd64.deb
     - ID: Ubu20P37b
       # ~25min
       PY: 3.7
@@ -198,7 +198,7 @@ environment:
       INSTALL_SYSPKGS: python3-virtualenv
       CODECOV_BINARY: https://uploader.codecov.io/latest/linux/codecov
       # system git-annex is way too old, use better one
-      INSTALL_GITANNEX: git-annex -m deb-url --url http://snapshot.debian.org/archive/debian/20210906T204127Z/pool/main/g/git-annex/git-annex_8.20210903-1_amd64.deb
+      INSTALL_GITANNEX: git-annex -m deb-url --url https://datasets.datalad.org/datalad/packages/neurodebian/git-annex_8.20210903-1_amd64.deb
 
 matrix:
   allow_failures: