scripts/installer.sh: check connectivity with pkgs.tailscale.com

Installer script relies on pkgs.tailscale.com being reachable, both for
checking what Linux distros are supported, but also for actually
downloading repo configuration files, gpg keys and packages themselves.

This change adds a simple reachability check which will print an error
message when pkgs.tailscale.com is not reachable.

Fixes https://github.com/tailscale/corp/issues/8952

Signed-off-by: Anton Tolchanov <anton@tailscale.com>

scripts/installer.sh

@@ -321,6 +321,17 @@ main() {
 		exit 1
 	fi
 
+	TEST_URL="https://pkgs.tailscale.com/"
+	RC=0
+	TEST_OUT=$($CURL "$TEST_URL" 2>&1) || RC=$?
+	if [ $RC != 0 ]; then
+		echo "The installer cannot reach $TEST_URL"
+		echo "Please make sure that your machine has internet access."
+		echo "Test output:"
+		echo $TEST_OUT
+		exit 1
+	fi
+
 	# Step 2: having detected an OS we support, is it one of the
 	# versions we support?
 	OS_UNSUPPORTED=