Draft: Bug 40060: Define versionName in Fenix build

Matthew Finkelrequested to merge
sysrqb/tor-browser-build:bug_40060_03 into master

In addition, we delete the hard-coded Fenix specific version and we use abbrev, instead. We add osname in the filename, as well, for consistency with the firefox build output.

(Small commit message modification from !83 (closed) )

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