fix cargo pkg version parsing

[therishidesai]

The setCargoCommonBuildEnv function in builder-common.sh doesn't properly use BASH_REMATCH to parse the version number. The 0 index of BASH_REMATCH is the whole string so the CARGO_PKG_VERSION_MAJOR was failing with an InvalidDigit Error. I noticed this when trying to package the ffmpeg-sys-next crate using nocargo.