By appending the old system PATH environment variable to the paths
setup by Termux system binaries are found as a fallback.
This causes problems with system binaries not working (due to
LD_LIBRARY_PATH) and causing a lot of confusion for new users when
e.g. an Android system provides a system version of e.g. curl, ssh
and other programs. It's better for these users to be prompted to
install the proper Termux package, and advanced users can still
add /system/bin to the PATH themselves.
Certain programs such as 'am' and 'pm' are already setup in
$PREFIX/bin to clear LD_LIBRARY_PATH and launch the binaries in
/system/bin - if there are some more popular ones they could be
added in the same way.
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