flow: preserve OPENROAD_EXE/OPENSTA_EXE origin across sub-makes

`export OPENROAD_EXE := $(OPENROAD_EXE)` rebinds the variable to
origin "file", which puts it on the UNSET_VARIABLES_NAMES list that
UNSET_AND_MAKE clears before invoking a sub-make. In sub-make the
variable is gone and the `?=` fallback resolves to the in-tree
`tools/install/OpenROAD/bin/openroad` path that does not exist in a
Bazel sandbox, so every stage invoked via UNSET_AND_MAKE (floorplan,
place, route, final) fails with "openroad: No such file or directory".

Using a bare `export` preserves the original origin (environment when
bazel-orfs supplies it, file when the local default fills in) so the
env value survives the UNSET_VARS step and sub-makes see the right
binary.

Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>

Author: oharboe

flow/scripts/variables.mk

@@ -92,8 +92,8 @@ else
   OPENSTA_EXE ?= $(abspath $(FLOW_HOME)/../tools/install/OpenROAD/bin/sta)
 endif
 
-export OPENROAD_EXE := $(OPENROAD_EXE)
-export OPENSTA_EXE  := $(OPENSTA_EXE)
+export OPENROAD_EXE
+export OPENSTA_EXE
 
 OPENROAD_IS_VALID := $(if $(OPENROAD_EXE),$(shell test -x $(OPENROAD_EXE) && echo "true"),)