New script build.sh is much more suitable for downstream packaging
(e.g. ./build.sh package) than hacked together package.sh wrapper for
get.sh while still being usable for "effort-less" installation.
Additionally, hostname setting in get.sh is flawed in many ways and is
not reimplemented in build.sh.
build.sh has proper command line options that allow to customize build
configuration and installation prefixes.
Documentation page get.sh is removed since all applicable environment
variables and flags are documented in ./build.sh --help.
build.sh can be called from the source directory to build maddy from
*this* source instead of forced 'go get' that was used in get.sh.
However, if build.sh is called not from the source directory, it clones
the repo and (optionally) uses the specified commit. This keeps build.sh
usable in curl|bash commands.
Due to the way source code is fetched, build.sh uses Git tags instead of
Go module versions as get.sh did.
