Ensure weekly Pulumi update job edits .pulumi.version

[t0yv0]

Since introducing https://github.com/pulumi/pulumi-command/pull/262 there is a .pulumi.version file that is especially relevant for the version of the SDK code generators used to produce language SDKs. This needs to be kept in sync and up-to-date. Ideally the weekly-pulumi-update.yml workflow can take care of it.

I copied the .pulumi.version trick from pulumi-azure-native but didn't work out how the weekly-pulumi-update works there, perhaps still ripe for borrowing. CC @danielrbradley