Link documentation to "Specifying environment variables for actions" design doc.
Link documentation to "Specifying environment variables for actions" design doc.

Fix #2730

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PiperOrigin-RevId: 151116555
MOS_MIGRATED_REVID=151116555

bazelbuild/bazel-website,bazelbuild/bazel-website,davidzchen/bazel-website,bazelbuild/bazel-website,davidzchen/bazel-website,davidzchen/bazel-website,davidzchen/bazel-website