Use context manager to restore environment variables in resolve()

[jdufresne]

In the event of an unexpected exception, the system's environment
variable PIP_EXISTS_ACTION should still be restored to its previous
state.

This context manager already exists in the upstream pip library. Can
reuse it to avoid duplication.

Unlike the previous implementation, if the PIP_EXISTS_ACTION environment
variable existed prior to executing pip-tools, that state will be
restored.

Contributor checklist

• Provided the tests for the changes.
• Gave a clear one-line description in the PR (that the maintainers can add to CHANGELOG.md on release).
• Assign the PR to an existing or new milestone for the target version (following Semantic Versioning).

[atugushev]

Nice! 👍