Separate installing and building self-hosted compiler

[BratishkaErik]

Regressed since e69973b, this can be a problem for package maintainers, for example https://bugs.gentoo.org/867592:

dev-lang/zig-9999: sandbox violation (tries to create /usr/lib/zig)

and https://aur.archlinux.org/packages/zig-git#comment-880012:

With the latest changes to the Zig build process the PKGBUILD no longer works. Problem is that the "cmake --build build" attempts to write directly to /usr/lib. The last part of the build uses zig (stage2) and wants to use -DCMAKE_INSTALL_PREFIX=/usr as written when building stage3. This is the second version of this PKGBUILD.

zig/CMakeLists.txt

Lines 1046 to 1065 in f7784a0

	set(ZIG_INSTALL_ARGS "build"
	--zig-lib-dir "${CMAKE_SOURCE_DIR}/lib"
	--prefix "${CMAKE_INSTALL_PREFIX}"
	"-Dconfig_h=${ZIG_CONFIG_H_OUT}"
	"-Denable-llvm"
	"-Denable-stage1"
	${ZIG_RELEASE_ARG}
	${ZIG_STATIC_ARG}
	${ZIG_SKIP_INSTALL_LIB_FILES_ARG}
	${ZIG_SINGLE_THREADED_ARG}
	"-Dtarget=${ZIG_TARGET_TRIPLE}"
	"-Dcpu=${ZIG_TARGET_MCPU}"
	)
	add_custom_target(stage3 ALL
	COMMAND zig2 ${ZIG_INSTALL_ARGS}
	DEPENDS zig2
	COMMENT STATUS "Building stage3"
	WORKING_DIRECTORY "${CMAKE_SOURCE_DIR}"
	)