feat(agent): add script data dir for binaries and files (#12205)

The agent is extended with a `--script-data-dir` flag, defaulting to the
OS temp dir. This dir is used for storing `coder-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

cli/testdata/coder_agent_--help.golden

@@ -33,6 +33,9 @@ OPTIONS:
       --prometheus-address string, $CODER_AGENT_PROMETHEUS_ADDRESS (default: 127.0.0.1:2112)
           The bind address to serve Prometheus metrics.
 
+      --script-data-dir string, $CODER_AGENT_SCRIPT_DATA_DIR (default: /tmp)
+          Specify the location for storing script data.
+
       --ssh-max-timeout duration, $CODER_AGENT_SSH_MAX_TIMEOUT (default: 72h)
           Specify the max timeout for a SSH connection, it is advisable to set
           it to a minimum of 60s, but no more than 72h.