devshells: update std to use latest direnv

`std` now exports the direnv lib to be easily sourced dynamically from
the source tree already fetched, rather than fetching again seperately.

Author: nrdxp

.envrc

@@ -6,7 +6,5 @@
 [ -f .envrc.local ] && source_env .envrc.local
 DEVSHELL_TARGET=${DEVSHELL_TARGET:-default}
 
-source_url \
-  "https://raw.githubusercontent.com/divnix/std/main/direnv_lib.sh" \
-  "sha256-JkRxy5PilGkOQcI/KaiZeyJKUTMTKduQwsw/ayUyz4k="
+. "$(nix eval .#__std.direnv_lib)"
 use std nix "//automation/devshells:${DEVSHELL_TARGET}"