package/micropython: fix building after dropping GIT_DIR=. workaround

The MICROPYTHON_MAKE_ENV variable contained two things;
  - the comon target environment variables CC, CFLAGS et al. defined in
    TARGET_MAKE_ENV,
  - the GIT_DIR workaround

Commit 9024e1866523 (package/micropython: drop GIT_DIR=. workaround)
totally dropped the assignment to MICROPYTHON_MAKE_ENV, but did not
replace its expansin with TARGET_MAKE_ENV.

This yields build error like:

    LINK build-standard/micropython
    arm-linux-gcc: ERROR: unsafe header/library path used in cross-compilation: '-L/usr/lib64/../lib64'

Fix this by expanding TARGET_MAKE_ENV in lieu of MICROPYTHON_MAKE_ENV.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

package/micropython/micropython.mk

@@ -37,13 +37,13 @@ MICROPYTHON_MAKE_OPTS += \
 	CWARN=
 
 define MICROPYTHON_BUILD_CMDS
-	$(MICROPYTHON_MAKE_ENV) $(MAKE) -C $(@D)/mpy-cross
-	$(MICROPYTHON_MAKE_ENV) $(MAKE) -C $(@D)/ports/unix \
+	$(TARGET_MAKE_ENV) $(MAKE) -C $(@D)/mpy-cross
+	$(TARGET_MAKE_ENV) $(MAKE) -C $(@D)/ports/unix \
 		$(MICROPYTHON_MAKE_OPTS)
 endef
 
 define MICROPYTHON_INSTALL_TARGET_CMDS
-	$(MICROPYTHON_MAKE_ENV) $(MAKE) -C $(@D)/ports/unix \
+	$(TARGET_MAKE_ENV) $(MAKE) -C $(@D)/ports/unix \
 		$(MICROPYTHON_MAKE_OPTS) \
 		DESTDIR=$(TARGET_DIR) \
 		PREFIX=/usr \