Set PATH of buildbox-casd to include bundled buildbox-fuse

Otherwise, buildbox-casd falls back to its HardLinkStager backend
which does not provide sufficient capabilities to build
gnome-build-meta.

See: #1733

src/buildstream/_cas/casdprocessmanager.py

@@ -35,6 +35,7 @@
 from .._protos.build.buildgrid import local_cas_pb2_grpc
 from .._protos.google.bytestream import bytestream_pb2_grpc
 
+from .. import _site
 from .. import utils
 from .._exceptions import CASCacheError
 
@@ -103,12 +104,29 @@ def __init__(self, path, log_dir, log_level, cache_quota, remote_cache_spec, pro
             # The frontend will take care of terminating buildbox-casd.
             # Create a new process group for it such that SIGINT won't reach it.
             self.process = subprocess.Popen(  # pylint: disable=consider-using-with, subprocess-popen-preexec-fn
-                casd_args, cwd=path, stdout=logfile_fp, stderr=subprocess.STDOUT, preexec_fn=os.setpgrp
+                casd_args,
+                cwd=path,
+                stdout=logfile_fp,
+                stderr=subprocess.STDOUT,
+                preexec_fn=os.setpgrp,
+                env=self.__buildbox_casd_env(),
             )
 
     def __buildbox_casd(self):
         return utils._get_host_tool_internal("buildbox-casd", search_subprojects_dir="buildbox")
 
+    def __buildbox_casd_env(self):
+        env = os.environ.copy()
+
+        # buildbox-casd needs to have buildbox-fuse in its PATH at runtime,
+        # otherwise it will fallback to the HardLinkStager backend.
+        bundled_buildbox_dir = os.path.join(_site.subprojects, "buildbox")
+        if os.path.exists(bundled_buildbox_dir):
+            path = env.get("PATH", "").split(os.pathsep)
+            path = [bundled_buildbox_dir] + path
+            env["PATH"] = os.pathsep.join(path)
+        return env
+
     # _check_casd_version()
     #
     # Check for minimal acceptable version of buildbox-casd.