Backport changes from get-tinypilot-pro.sh (#1880)

Resolves #1878

This PR backports the relevant changes to `get-tinypilot-pro.sh`
(introduced by tiny-pilot/gatekeeper#301) back
into `get-tinypilot.sh`.

<a data-ca-tag
href="https://codeapprove.com/pr/tiny-pilot/tinypilot/1880"><img
src="https://codeapprove.com/external/github-tag-allbg.png" alt="Review
on CodeApprove" /></a>

Author: jdeanwallace

get-tinypilot.sh

@@ -126,7 +126,7 @@ readonly INSTALLER_DIR
 # that we take advantage of RAMdisk if we're using one.
 readonly TMPDIR="${INSTALLER_DIR}/tmp"
 export TMPDIR
-sudo mkdir "${TMPDIR}"
+sudo mkdir --mode=1777 "${TMPDIR}"
 
 #######################################
 # Executes a curl command that returns a non-zero exit code based on the HTTP