Use a more specific condition for github actions matching

[rcarver]

Updates to the CI/env issue

• Confirmed Update Package.swift to use GitHub Action env var #237 fails on my CI
• Confirmed this branch succeeds
• Confirm that this branch correctly builds docs (I don't know how to do that)

What's strange is that the package that directly depends on swift-structured-queries builds ok, but one two hops away fails. The dependency tree looks like:

app-package
  core-package
    swift-structured-queries

in app-package and core-package there's an Xcode workspace used to build and test for CI. This is the simplest command to reproduce the failure. This command was run locally with cleaned DerivedData

# in app-package
GITHUB_ACTION=1 xcodebuild -resolvePackageDependencies
xcodebuild: error: Could not resolve package dependencies:
  product 'CasePaths' required by package 'swift-structured-queries' target 'StructuredQueriesCore' not found in package 'swift-case-paths'.

# in core-package
GITHUB_ACTION=1 xcodebuild -resolvePackageDependencies
success!