buildFHSEnv: fix substring match incorrectly skipping host directories

The auto-mount loop used bash regex =~ which does substring matching, so
top-level directories whose names are prefixes of FHS paths (e.g. /sb
matches /sbin) were silently skipped instead of bind-mounted into the
chroot. Switch to associative array for exact-match lookup.

Fixes #241151

pkgs/build-support/build-fhsenv-bubblewrap/default.nix

@@ -222,9 +222,14 @@ let
         ro_mounts+=(--ro-bind /etc /.host-etc)
       fi
 
+      declare -A etc_ignored_set
+      for ign in "''${etc_ignored[@]}"; do
+        etc_ignored_set[$ign]=1
+      done
+
       # link selected etc entries from the actual root
       for i in ${escapeShellArgs etcBindEntries}; do
-        if [[ "''${etc_ignored[@]}" =~ "$i" ]]; then
+        if [[ -n "''${etc_ignored_set[$i]:-}" ]]; then
           continue
         fi
         if [[ -e $i ]]; then
@@ -232,11 +237,21 @@ let
         fi
       done
 
+      declare -A ignored_set
+      for ign in "''${ignored[@]}"; do
+        ignored_set[$ign]=1
+      done
+
       declare -a auto_mounts
       # loop through all directories in the root
       for dir in /*; do
-        # if it is a directory and it is not ignored
-        if [[ -d "$dir" ]] && [[ ! "''${ignored[@]}" =~ "$dir" ]]; then
+        # if it is a directory and not already provided by the FHS env or
+        # explicitly ignored, bind-mount it into the chroot. Use exact match
+        # via associative array because regex substring matching incorrectly
+        # skips prefixes (e.g. /sb would match /sbin and never get mounted,
+        # breaking --chdir when CWD is on a custom mount like /sb/project).
+        # https://github.com/NixOS/nixpkgs/issues/241151
+        if [[ -d "$dir" ]] && [[ -z "''${ignored_set[$dir]:-}" ]]; then
           # add it to the mount list
           auto_mounts+=(--bind "$dir" "$dir")
         fi