Copy the value to our `ConfigName` member on construction to ensure it
is available even to code paths that run before `Generate`.  We once
needed to delay this lookup until `Generate` because the local
generators were at one time created before `Configure`.  Now they are
created at generate time which is late enough to expect
`CMAKE_BUILD_TYPE` to be available.

Without this, `cmGlobalUnixMakefileGenerator3::WriteConvenienceRules`
causes use of `ConfigName` before it is populated which breaks use of
source files that depend on the `$<CONFIG>` generator expression.

Fixes: #16889