Changes to `swift.swiftEnvironmentVariables` require a restart before they're reflected in the build tasks

[plemarquand]

Built in built tasks (Debug MyProject, Release MyProject) are created once, and are not updated when the user changes the swift.swiftEnvironmentVariables setting. They must restart the extension before the build task is updated with the new environment variable(s).

[award999]

We prompt to restart but overall should handle better