Make the post-build-hook use the daemon Nix package

Having the `post-build-hook` use `nix` from the client package can lead
to a deadlock in case there’s a db migration to do between both, as a
`nix` command running inside the hook will run as root (and as such will
bypass the daemon), so might trigger a db migration, which will get
stuck trying to get a global lock on the DB (as the daemon that ran the
hook already has a lock on it).

(cherry picked from commit 93eadd5)

tests/common.sh.in

@@ -33,8 +33,9 @@ export PATH=@bindir@:$PATH
 if [[ -n "${NIX_CLIENT_PACKAGE:-}" ]]; then
   export PATH="$NIX_CLIENT_PACKAGE/bin":$PATH
 fi
+DAEMON_PATH="$PATH"
 if [[ -n "${NIX_DAEMON_PACKAGE:-}" ]]; then
-  export NIX_DAEMON_COMMAND="$NIX_DAEMON_PACKAGE/bin/nix-daemon"
+  DAEMON_PATH="${NIX_DAEMON_PACKAGE}:$DAEMON_PATH"
 fi
 coreutils=@coreutils@
 
@@ -87,7 +88,7 @@ startDaemon() {
     # Start the daemon, wait for the socket to appear.  !!!
     # ‘nix-daemon’ should have an option to fork into the background.
     rm -f $NIX_DAEMON_SOCKET_PATH
-    ${NIX_DAEMON_COMMAND:-nix-daemon} &
+    PATH=$DAEMON_PATH nix-daemon &
     for ((i = 0; i < 30; i++)); do
         if [[ -S $NIX_DAEMON_SOCKET_PATH ]]; then break; fi
         sleep 1