[release] Allow build version to be injected at compile time

[clintjedwards]

This adds a new build envvar that will overwrite the default CARGO_PKG_VERSION and removes some of the logic we were using to generate unique build versions for dev versions.

In short:

• We introduce a new PUSHPIN_BUILD_VERSION environment variable (prefixed to not collide with CI env vars) that we use whenever we want to overwrite the build version listed in cargo.toml.
• Building the normal way (cargo build --release) without any extra envvars still uses the version in cargo.toml as the version.
• This allows us some flexibility when building final assets as to what the version string actually gets set as. Very useful when hotfixing, for downstream forks, or just generally doing things that need the version string to change but don't want cargo.toml to be updated. (Things like appending the date to the build version and such)
• This PR also cleans up the -dev logic for appending dates since we're moving away from explicitly tagging out of band release builds with a -dev suffix.
  • The env var addition actually becomes a mechanism where we can reinstate this behavior if wanted to. For example, explicitly setting the env var would set the version string as normal, but not setting it could append some identifier like -dev or -rc or whatever (Or vice-versa).