Consolidate configuration of stmgr version

The version of stmgr is set in several different places:

• https://git.glasklar.is/system-transparency/core/stimages/-/blob/main/build-stmgr#L8 uses a variable STMGR_VERSION for the default, set to v0.4.1. It is overridable by a command line argument $1.
• Only use of this script (at least in this repo) is at https://git.glasklar.is/system-transparency/core/stimages/-/blob/main/build-stimage#L16, explicitly passing "latest" as the version.
• The build-stboot script, https://git.glasklar.is/system-transparency/core/stimages/-/blob/main/contrib/stboot/build-stboot#L12 uses an environment variable STMGR_VERSION, with a default of v0.5.0 (I bumped this recently, as part of the stboot release work). This script uses the hostconfig, trustpolicy and uki subcommands.
• The getting-started script, https://git.glasklar.is/system-transparency/core/stimages/-/blob/main/test/getting-started.sh#L20 uses a different environment variable, STMGRVER, and a default of latest. It uses the keygen and ospkg subcommands. It also invokes the above build-stboot scripts, unclear to me if they use the same install location for their two distinct versions of stmgr.