Switch devenv from development branch to v0.6.3

Broke in https://github.com/matrix-org/synapse/pull/16019. This would have been caught
if someone ran 'devenv up' during manual testing of that PR, or better yet, if we had
CI for the nix developer environment.

Switch back to the latest release version, which doesn't have the upstream issue:
https://github.com/cachix/devenv/issues/756

flake.lock

@@ -8,16 +8,16 @@
         "pre-commit-hooks": "pre-commit-hooks"
       },
       "locked": {
-        "lastModified": 1690534632,
-        "narHash": "sha256-kOXS9x5y17VKliC7wZxyszAYrWdRl1JzggbQl0gyo94=",
+        "lastModified": 1688058187,
+        "narHash": "sha256-ipDcc7qrucpJ0+0eYNlwnE+ISTcq4m03qW+CWUshRXI=",
         "owner": "cachix",
         "repo": "devenv",
-        "rev": "6568e7e485a46bbf32051e4d6347fa1fed8b2f25",
+        "rev": "c8778e3dc30eb9043e218aaa3861d42d4992de77",
         "type": "github"
       },
       "original": {
         "owner": "cachix",
-        "ref": "main",
+        "ref": "v0.6.3",
         "repo": "devenv",
         "type": "github"
       }

flake.nix

@@ -45,7 +45,7 @@
     # Output a development shell for x86_64/aarch64 Linux/Darwin (MacOS).
     systems.url = "github:nix-systems/default";
     # A development environment manager built on Nix. See https://devenv.sh.
-    devenv.url = "github:cachix/devenv/main";
+    devenv.url = "github:cachix/devenv/v0.6.3";
     # Rust toolchain.
     rust-overlay.url = "github:oxalica/rust-overlay";
   };