The agent is extended with a--script-data-dir flag, defaulting to the
OS temp dir. This dir is used for storingcoder-script-data/bin and
coder-script/[script uuid]. The former is a place for all scripts to
place executable binaries that will be available by other scripts, SSH
sessions, etc. The latter is a place for the script to store files.

Since we default to OS temp dir, files are ephemeral by default. In the
future, we may consider adding new env vars or changing the default
storage location. Workspace startup speed could potentially benefit from
scripts being able to skip steps that require downloading software. We
may also extend this with more env variables (e.g. persistent storage in
HOME).

Fixes#11131

This can be feature detected by checking if$CODER_SCRIPT_DATA_DIR and/or$CODER_SCRIPT_BIN_DIR is set.