contrib: Allow cmake make invocation to communicate with the jobserver

Since make can't ensure $(CMAKEBUILD) will invoke $(MAKE) itself, the sub-make process wasn't able to communicate with the jobserver, causing jobs to run with a single job instead of however much are available.

contrib/src/main.mak

@@ -439,7 +439,7 @@ RECONF = mkdir -p -- $(PREFIX)/share/aclocal && \
 # When using a single command, make might take a shortcut and fork/exec
 # itself instead of relying on a shell, but a bug in gnulib ends up
 # trying to execute a cmake folder when one is found in the PATH
-CMAKEBUILD := $(shell command -v cmake) --build
+CMAKEBUILD := env cmake --build
 CMAKE = cmake . -DCMAKE_TOOLCHAIN_FILE=$(abspath toolchain.cmake) \
 		-DCMAKE_INSTALL_PREFIX:STRING=$(PREFIX) \
 		-DBUILD_SHARED_LIBS:BOOL=OFF