ci: errno: add required ZEPHYR_SDK_INSTALL_DIR env var

ZEPHYR_SDK_INSTALL_DIR is a required environment variable, otherwise the
script fails.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>

.github/workflows/errno.yml

@@ -9,6 +9,8 @@ jobs:
     runs-on: ubuntu-latest
     container:
       image: zephyrprojectrtos/ci:v0.21.0
+    env:
+      ZEPHYR_SDK_INSTALL_DIR: /opt/toolchains/zephyr-sdk-0.13.2
 
     steps:
       - name: checkout