Expose GITHUB_ACTIONS env var to docker containers (#3123)

### Motivation:

It can be useful for scripts to be able to check if they are being run
in CI. Because we invoke docker on the CI runners, the env var which
people may use to do this isn't present.

### Modifications:

Expose `GITHUB_ACTIONS` env var to docker containers in the matrix
workflow

### Result:

Adopters of the matrix workflow or downstream workflows will be able to
check if they are running in CI

.github/workflows/swift_test_matrix.yml

@@ -43,6 +43,7 @@ jobs:
           workspace="/$(basename ${{ github.workspace }})"
           docker run -v ${{ github.workspace }}:"$workspace" \
             -w "$workspace" \
+            -e GITHUB_ACTIONS="$GITHUB_ACTIONS" \
             -e SWIFT_VERSION="${{ matrix.config.swift_version }}" \
             -e setup_command_expression="$setup_command_expression" \
             -e workspace="$workspace" \
@@ -59,6 +60,7 @@ jobs:
           $workspace = "C:\" + (Split-Path ${{ github.workspace }} -Leaf)
           docker run -v ${{ github.workspace }}:$($workspace) `
             -w $($workspace) `
+            -e GITHUB_ACTIONS=%GITHUB_ACTIONS% `
             -e SWIFT_VERSION="${{ matrix.config.swift_version }}" `
             -e setup_command_expression=%setup_command_expression% `
             ${{ matrix.config.image }} `