Minor: Configuring with multiple RTEMS targets

[anjohnson]

A GHA build configured with

CI_CROSS_TARGETS="RTEMS-beatnik@5:RTEMS-xilinx_zynq_a9_qemu@5:RTEMS-uC5282@5"

Seems to have built all 3 targets as I hoped it would, but during the cue.py prepare step it output this:

Configuring EPICS build system
  EPICS Base build system set up for optimized build with shared (DLL) linking
  Cross compiler RTEMS5 @ RTEMS-beatnik
  Downloading RTEMS 5 cross compiler: beatnik-rtems5.tar.xz
  /opt/rtems/5/bin/powerpc-rtems5-gcc --version
  powerpc-rtems5-gcc (GCC) 7.5.0 20191114 (RTEMS 5, RSB 5 (3911854[46](https://github.com/anjohnson/pvDataCPP/actions/runs/10337027940/job/28613430850#step:5:49)2e7 modified), Newlib 7947581)
  Copyright (C) 2017 Free Software Foundation, Inc.
  This is free software; see the source for copying conditions.  There is NO
  warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
  
  Cross compiler RTEMS5 @ RTEMS-xilinx_zynq_a9_qemu
  Downloading RTEMS 5 cross compiler: xilinx_zynq_a9_qemu-rtems5.tar.xz
  /opt/rtems/5/bin/arm-rtems5-gcc --version
  arm-rtems5-gcc (GCC) 7.5.0 20191114 (RTEMS 5, RSB 5 (3911854462e7 modified), Newlib 7947581)
  Copyright (C) 2017 Free Software Foundation, Inc.
  This is free software; see the source for copying conditions.  There is NO
  warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
  
  /opt/rtems/5/bin/powerpc-rtems5-gcc --version
  powerpc-rtems5-gcc (GCC) 7.5.0 20191114 (RTEMS 5, RSB 5 (3911854462e7 modified), Newlib 7947581)
  Copyright (C) 2017 Free Software Foundation, Inc.
  This is free software; see the source for copying conditions.  There is NO
  warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
  
  Cross compiler RTEMS5 @ RTEMS-uC[52](https://github.com/anjohnson/pvDataCPP/actions/runs/10337027940/job/28613430850#step:5:55)82
  Downloading RTEMS 5 cross compiler: uC5282-rtems5.tar.xz
  /opt/rtems/5/bin/m68k-rtems5-gcc --version
  m68k-rtems5-gcc (GCC) 7.5.0 20191114 (RTEMS 5, RSB 5 (39118[54](https://github.com/anjohnson/pvDataCPP/actions/runs/10337027940/job/28613430850#step:5:57)462e7 modified), Newlib 7947581)
  Copyright (C) 2017 Free Software Foundation, Inc.
  This is free software; see the source for copying conditions.  There is NO
  warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
  
  /opt/rtems/5/bin/arm-rtems5-gcc --version
  arm-rtems5-gcc (GCC) 7.5.0 20191114 (RTEMS 5, RSB 5 (3911854462e7 modified), Newlib 7947[58](https://github.com/anjohnson/pvDataCPP/actions/runs/10337027940/job/28613430850#step:5:61)1)
  Copyright (C) 2017 Free Software Foundation, Inc.
  This is free software; see the source for copying conditions.  There is NO
  warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
  
  /opt/rtems/5/bin/powerpc-rtems5-gcc --version
  powerpc-rtems5-gcc (GCC) 7.5.0 20191114 (RTEMS 5, RSB 5 (39118544[62](https://github.com/anjohnson/pvDataCPP/actions/runs/10337027940/job/28613430850#step:5:65)e7 modified), Newlib 7947581)
  Copyright (C) 2017 Free Software Foundation, Inc.
  This is free software; see the source for copying conditions.  There is NO
  warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

So it downloaded each compiler just once, but ran the gcc --version commands for all of the installed cross-compilers in reverse order after installing each one.

It might be worth putting a check in the code when multiple RTEMS targets are built to ensure that they all specify the same RTEMS version. It's not possible to mix RTEMS versions in a single Base build tree.