build_config: give the msvc build a name of its own - #7246

build_config/ci/msvc.rb opened an anonymous `MRuby::Build`, and an
unnamed build is called 'host' (lib/mruby/build.rb:114).  The build tree
is keyed by that name (:127), so MRUBY_CONFIG=ci/msvc aimed a full-core
build with `MRB_GC_FIXED_ARENA` at build/host, the directory a default
build on the same machine uses.  The Windows-VC job never sees that,
since it builds a fresh checkout, but a developer who points
MRUBY_CONFIG at this config on a tree the default config has built
shares one directory between two gem sets, which is the shape mruby#7195,
mruby#7197, mruby#7199 and mruby#7200 took out of the other configs that had it.

14d6e42 named the bintest build in ci/gcc-clang and left this one alone,
on the ground that a single build has no sibling for a name to tell it
apart from.  That was a reason about telling builds apart; the build
directory is decided by the name whether or not there is a sibling, and
that is the reason here.

Name it after its config, as build_config/mrbc.rb, clang-asan.rb and
gctest.rb are.  Nothing in the file needs the build to be called
'host'.  It is an `MRuby::Build`, and it pulls `mruby-bin-mrbc` in
through full-core, so it lends no mrbc to any cross target and creates
its own internal one either way (:174).  The name reaches three other
places: `install_prefix` (:625), the `bin/*.bat` wrappers that
`define_installer_if_needed` writes only when `host?` answers true (:461
and :482), and the mrbtest that `rake clean` removes from MRUBY_ROOT/bin
(tasks/test.rake:82).  The job runs `rake -m test:run:serial`
(.github/workflows/build.yml:89), which installs nothing and cleans
nothing, so none of the three fires.  `enable_bintest` has not required
the name 'host' since fd113da, and test/bintest.rb resolves every binary
it runs from BUILD_DIR rather than from bin/; the `bintest` build in
ci/gcc-clang is this same shape under a name, in the same workflow.

There is no MSVC here to run the job with.  What can be checked on
Linux is that the config still loads and that the tree it names is its
own; the job on this change is the run:

    $ MRUBY_CONFIG=ci/msvc rake -T | grep amalgam
    rake amalgam    # Generate amalgamated mruby.h and mruby.c in .../build/msvc/amalgam