Re-evaluate adding Bootsnap to GitLab

We had tried enabling bootsnap previously:

• https://gitlab.com/gitlab-org/gitlab-ce/issues/33081

We also found few issues with it:

• https://gitlab.com/gitlab-org/gitlab-ce/issues/34106
• https://gitlab.com/gitlab-org/gitlab-ce/issues/34799
• https://github.com/Shopify/bootsnap/issues/67#issuecomment-313451127

While the ARMV7 issue is still not fixed, we could get some improvements while in GDK and/or production.

My idea is to use an ENV variable (like: DISABLE_BOOTSNAP=1) to disable bootsnap on our RPi builds, but let it on for everything else. We keep this kill-switch available in order to reduce risks.

cc @stanhu @WarheadsSE @rspeicher