ci: convert "install-dependencies.sh" to use "/bin/sh"

ci: convert "install-dependencies.sh" to use "/bin/sh"

We're about to merge the "install-docker-dependencies.sh" script into
"install-dependencies.sh". This will also move our Alpine-based jobs
over to use the latter script. This script uses the Bash shell though,
which is not available by default on Alpine Linux.

Refactor "install-dependencies.sh" to use "/bin/sh" instead of Bash.
This requires us to get rid of the pushd/popd invocations, which are
replaced by some more elaborate commands that download or extract
executables right to where they are needed.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>