trees: more robust check to avoid "make fetch"

do not use shorthand here. the test was failing to
produce the desired result under some circumstances,
for example when i did "./update release" i got this:

make: Entering directory '/home/lbdev/lbmk/release/20240612-62-ga6b1a6bd/libreboot-20240612-62-ga6b1a6bd_src/src/stm32-vserprog'
make: *** No rule to make target 'fetch'.  Stop.
make: Leaving directory '/home/lbdev/lbmk/release/20240612-62-ga6b1a6bd/libreboot-20240612-62-ga6b1a6bd_src/src/stm32-vserprog'
ERROR script/trees: !mk src/stm32-vserprog fetch
ERROR ./update: excmd: script/trees -f
ERROR script/roms: Unhandled non-zero exit: ./update
ERROR ./build: excmd: script/roms serprog
ERROR ./update: build_release release/20240612-62-ga6b1a6bd: stm32
ERROR ./update: can't build rom images

in the above circumstance, run_make_command was executed,
which is not the desired behaviour; rather, fetch_project_trees
or fetch_project_repo should be called, and then the script
should immediately exit. it should also exit, without downloading
anything, if a changelog file exists as in release archives.

Signed-off-by: Leah Rowe <leah@libreboot.org>

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