bionic-amd64-tf-m-build: Fix copy-paste mistake for ArmClang 6.18

Due to missing to update path in one place, the installer for older
ArmClang version (6.13) was run, and installed into 6.18 directory.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: I4c089b6c2371c6f254450bdfb507e2136f8e1214
1 file changed