fix (internal/civisibility): fix travis environment variables mapping

[tonyredondo]

What does this PR do?

Fixes the environment variables mapping for Travis CI Provider.

Motivation

from travis docs:

TRAVIS_PULL_REQUEST_SHA:
if the current job is a pull request, the commit SHA of the HEAD commit of the PR. NOT THE BASE SHA

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• New code is free of linting errors. You can check this by running golangci-lint run locally.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.

Unsure? Have a question? Request a review!

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-06-02 10:23:43

Comparing candidate commit 71a0f59 in PR branch tony/fix-travis-env-vars with baseline commit 2635a5b in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 21 metrics, 0 unstable metrics.