Update build-using-self (#6594)

For local testing, it is important to also do a package update in the integration tests package.

Author: neonichu

Utilities/build-using-self

@@ -31,4 +31,6 @@ swift test -c $CONFIGURATION --parallel
 # Run the integration tests with just built SwiftPM.
 export SWIFTPM_BIN_DIR=$(swift build -c $CONFIGURATION --show-bin-path)
 cd IntegrationTests
+# Perform package update in order to get the latest commits for the dependencies.
+swift package update
 $SWIFTPM_BIN_DIR/swift-test --parallel