Makefile: set install target as phony

`install/` directory results in make install target always up to date.
Mark `install` as phony to be able to run it.

Replace `cmd/flux` with `./cmd/flux` for install to work and add
`CGO_ENABLED=0` like in other build and install targets.

Signed-off-by: Sunny <darkowlzz@protonmail.com>