ci: clear S3_JWT_FILE_SCRIPT as it also contains the S3_JWT

This whole thing of dumping the env var in a file and unsetting it so
that it wouldn't be visible in an env dump anymore?  Yeah, we kinda
failed here 😅

Note: setting it to an empty string instead of unsetting it allows for
redundant `eval "$S3_JWT_FILE_SCRIPT"` calls without failing over an
unset variable.

Reported-by: @alatiera
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/35050>

.gitlab-ci.yml

@@ -87,6 +87,7 @@ variables:
   S3_JWT_FILE: /s3_jwt
   S3_JWT_FILE_SCRIPT: |-
       echo -n '${S3_JWT}' > '${S3_JWT_FILE}' &&
+      S3_JWT_FILE_SCRIPT= &&
       unset CI_JOB_JWT S3_JWT  # Unsetting vulnerable env variables
   S3_HOST: s3.freedesktop.org
   # This bucket is used to fetch ANDROID prebuilts and images