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: 5f73e7d0386d ("kbuild: refactor cross-compiling linux-headers package")
Co-developed-by: Mingcong Bai <[e-mail]>
Signed-off-by: Mingcong Bai <[e-mail]>
Tested-by: WangYuli <[e-mail]>
Signed-off-by: WangYuli <[e-mail]>
Signed-off-by: Masahiro Yamada <[e-mail]>

scripts/package/install-extmod-build

···
 	# 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