Build S3 cache session policy with encoding/json

See merge request https://gitlab.com/gitlab-org/security/gitlab-runner/-/merge_requests/105



Merged-by: Axel von Bertoldi <avonbertoldi@gitlab.com>
Approved-by: Axel von Bertoldi <avonbertoldi@gitlab.com>
Co-authored-by: Stan Hu <stanhu@gmail.com>
(cherry picked from commit 7a6a41cbe2f993617a2ba19de24ab5740ab9de7a)
(cherry picked from commit 77a79ca8)

Merge branch 'sh-fix-role-arn-injection' into '18-11-stable'

See merge request !6715



Merged-by: Axel von Bertoldi <avonbertoldi@gitlab.com>
Approved-by: Romuald Atchadé <ratchade@gitlab.com>