Always set CGO_ENABLED=0

[fujiwara]

Hi!

The latest release binaries seems to be build dynamic linked.
For example https://github.com/mackerelio/mkr/releases/download/v0.55.0/mkr_linux_amd64.tar.gz

$ curl -sLO https://github.com/mackerelio/mkr/releases/download/v0.55.0/mkr_linux_amd64.tar.gz
$ tar xvf mkr_linux_amd64.tar.gz
$ ldd mkr_linux_amd64/mkr
linux-vdso.so.1 (0x00007ffe0359c000)
        libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f6c654a7000)
        /lib64/ld-linux-x86-64.so.2 (0x00007f6c656de000)

But, the mkr binary in Debian packages are statically linked.

$ ldd /usr/bin/mkr
        not a dynamic executable

I hope to use the release binaries in non-glibc environments (like Alpine Linux).

Currently, CGO_ENABLED=0 is set only when building via 'make build'. But the setting does not affect 'make cross', so release binaries seems dynamically linked.

This PR sets CGO_ENABLED=0 always.

refs #515

[lufia]

Understanding.

CGO_ENABLED is set to 1 by default when both build host's GOOS and GOARCH is same with target's them.

[lufia]

Thank you!