Fix static file generation on CI

[CristhianMotoche]

I noticed that the last release included the dotenv binary but the file isn't static:

$ tar xf dotenv-linux-x86_64-static.tar.gz
$ file dotenv
dotenv: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 3.2.0, BuildID[sha1]=d992f00e7630549de68fd7390d5b4db81aefd25d, with debug_info, not stripped

I suggest to take the hapistrano draft as an example. If the issue can be fixed in a different way, that's okay.