Add an `INSTALL_JOB_SERVER_AWARE` option to `ExternalProject_Add`.
When using an explicit `INSTALL_COMMAND`, the generated commands
won't use `$(MAKE)` thus failing to connect to the outer make's
job server. Add an option enable explicit job server integration.
This is the install step's equivalent to the build step's
`BUILD_JOB_SERVER_AWARE` option added by commit bc43398e72
(ExternalProject: Enable Make Job Server with Explicit Build Command,
2023-08-09, v3.28.0-rc1~217^2). It is useful when the external
project's installation is driven by its build system. Note that with
Makefile generators, our default install command does use `$(MAKE)` to
connect to the outer make's job server.
Issue: #26398
1bdaad7573