Only set ALUMET_AGENT_RELEASE=true for stable, released versions

[TheElectronWill]

ALUMET_AGENT_RELEASE=true is now hard-coded in the compilation step of the packages, it should not.

What we want:

• If we are building against a stable, released version (i.e. there is a tag or github release associated to the commit) => set ALUMET_AGENT_RELEASE=true
• Otherwise, we are building against a development version => set ALUMET_AGENT_RELEASE=false

Open for discussion: should we build in debug mode when not in release? Or add another parameter for that?