build/windows: Always use latest commit in luajit and lua-lgi

luajit download server to tarballs stopped to work, which is expected,
since it was advised by the team in https://luajit.org/download.html:

"Please do not use obsolete versions from older tarballs or zip files.
Please remove any outdated links to these downloads — these links will
cease to work soon."

---

lua-lgi, on the other hand, is mostly abandonware right now, so it's
safe to use latest master. See: lgi-devs/lgi#225

build/linux/flatpak/org.gimp.GIMP-nightly.json

@@ -658,15 +658,9 @@
             "name": "luajit",
             "sources": [
                 {
-                    "type": "archive",
-                    "url": "https://luajit.org/download/LuaJIT-2.1.0-beta3.tar.gz",
-                    "sha256": "1ad2e34b111c802f9d0cdf019e986909123237a28c746b21295b63c9e785d9c3",
-                    "x-checker-data": {
-                        "type": "anitya",
-                        "project-id": 6444,
-                        "stable-only": false,
-                        "url-template": "https://luajit.org/download/LuaJIT-$version.tar.gz"
-                    }
+                    "type": "git",
+                    "url": "https://github.com/LuaJIT/LuaJIT.git",
+                    "branch": "v2.1"
                 },
                 {
                     "type": "shell",
@@ -679,9 +673,6 @@
             "make-args": [
                 "DEFAULT_CC=ccache clang"
             ],
-            "post-install": [
-                "ln -s /app/bin/luajit-2.1.0-beta3 /app/bin/luajit"
-            ],
             "cleanup": [
                 "/include",
                 "/lib/pkgconfig",
@@ -693,8 +684,8 @@
             "sources": [
                 {
                     "type": "git",
-                    "url": "https://github.com/pavouk/lgi.git",
-                    "commit": "c21f35fccae87fd4e3625d4c878f584b7255d6f6"
+                    "url": "https://github.com/lgi-devs/lgi.git",
+                    "branch": "master"
                 }
             ],
             "buildsystem": "meson",