kbuild: install-extmod-build: add missing quotation marks for CC variable

While attempting to build a Debian packages with CC="ccache gcc", I
saw the following error as builddeb builds linux-headers-$KERNELVERSION:

  make HOSTCC=ccache gcc VPATH= srcroot=. -f ./scripts/Makefile.build obj=debian/linux-headers-6.14.0-rc1/usr/src/linux-headers-6.14.0-rc1/scripts
  make[6]: *** No rule to make target 'gcc'.  Stop.

Upon investigation, it seems that one instance of $(CC) variable reference
in ./scripts/package/install-extmod-build was missing quotation marks,
causing the above error.

Add the missing quotation marks around $(CC) to fix build.

Fixes: 5f73e7d0 ("kbuild: refactor cross-compiling linux-headers package")
Co-developed-by: Mingcong Bai <jeffbai@aosc.io>
Signed-off-by: Mingcong Bai <jeffbai@aosc.io>
Tested-by: WangYuli <wangyuli@uniontech.com>
Signed-off-by: WangYuli <wangyuli@uniontech.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/package/install-extmod-build

@@ -63,7 +63,7 @@ if [ "${CC}" != "${HOSTCC}" ]; then
 	# Clear VPATH and srcroot because the source files reside in the output
 	# directory.
 	# shellcheck disable=SC2016 # $(MAKE), $(CC), and $(build) will be expanded by Make
-	"${MAKE}" run-command KBUILD_RUN_COMMAND='+$(MAKE) HOSTCC=$(CC) VPATH= srcroot=. $(build)='"${destdir}"/scripts
+	"${MAKE}" run-command KBUILD_RUN_COMMAND='+$(MAKE) HOSTCC="$(CC)" VPATH= srcroot=. $(build)='"${destdir}"/scripts
 
 	rm -f "${destdir}/scripts/Kbuild"
 fi