[build infra] disable opam sandboxing for ubuntu focal

[jimexist]

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disable opam sandboxing for ubuntu focal, in order to fix error message https://app.travis-ci.com/github/apache/thrift/jobs/583252815

• [x] Did you create an Apache Jira ticket? (not required for trivial changes)
• [x] If a ticket exists: Does your pull request title follow the pattern "THRIFT-NNNN: describe my issue"?
• [x] Did you squash your changes to a single commit? (not required, but preferred)
• [x] Did you do your best to avoid breaking changes? If one was needed, did you label the Jira ticket with "Breaking-Change"?
• [x] If your change does not involve any code, include [skip ci] anywhere in the commit message to free up build resources.

[jimexist]

i see that this build went through successfully, so maybe this error is transient and thus requires no fix

[jimexist]

Hi @Jens-G I'm trying to resolve the opam error by disabling sandboxing which isn't needed given it's already run within a docker image.

this will expose other failures in swift but it can be fixed in a subsequent PR.

[jimexist]

i'm merging this in hope for fixing ocaml broken build