ensure git is not installed by package manager

since we're gonna compile it

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>
git - upgrade to v2.0.4

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>
[BUGFIX] set `ignore_errors` to true for checking if git tarball's extracted

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>
[BUGFIX] update-etc-environment-path.py - extract string from return value

Otherwise we will be dumping the string representation of a tuple into
the `/etc/environment` file.

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>
[BUGFIX] install_golang.yml - take into account of Mac OS X checksum

for the `update-etc-environment.py` script, since Mac OS X doesn't seem
to have a `sha256sum` program.

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>
install_golang.yml - ensure that SHA256sum of update-etc-environment script is taken

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>