ci: Add missing environment variable to integration-test

[simonjbeaumont]

Motivation

When we migrated the CI to Github Actions we lost an important configuration for the integration test that makes sure we're using the copy of this repo from the PR, rather than cloning from upstream. This can be seen in the logs of recent PR jobs:

** Cloning https://github.com/apple/swift-openapi-generator to /tmp/run-integration-test.sh.SUVBe45RMQ/swift-openapi-generator
Cloning into '/tmp/run-integration-test.sh.SUVBe45RMQ/swift-openapi-generator'...
** Extracting name for Swift package: /__w/swift-openapi-generator/swift-openapi-generator
** Overriding dependency in /tmp/run-integration-test.sh.SUVBe45RMQ/swift-openapi-generator/IntegrationTest on swift-openapi-generator to use /__w/swift-openapi-generator/swift-openapi-generator

This used to be specified in the Docker Compose file as an environment variable but dropped off during migration.

Modifications

Add the environment variable back to use file://${GITHUB_WORKSPACE} for the clone.

Note that one cannot use the env: map when making use of a reusable GitHub workflow with uses: so it's been added in the command passed to the matrix job, itself.

Result

The integration test CI will use this repo for the test itself so we can make sure we're testing the right thing.

Test Plan

The logs on this PR job show that we're now cloning from the GitHub workspace:

** Cloning file:///__w/swift-openapi-generator/swift-openapi-generator to /tmp/run-integration-test.sh.OWgemDungm/swift-openapi-generator
Cloning into '/tmp/run-integration-test.sh.OWgemDungm/swift-openapi-generator'...
Note: switching to '9664e9cb2170a5fa8a1bad3266891d4b4248e6ba'.
...
** Extracting name for Swift package: /__w/swift-openapi-generator/swift-openapi-generator
** Overriding dependency in /tmp/run-integration-test.sh.OWgemDungm/swift-openapi-generator/IntegrationTest on swift-openapi-generator to use /__w/swift-openapi-generator/swift-openapi-generator

[czechboy0]

Thanks for spotting and fixing this! 🙏