Add CMAKE_FIND_USE_INSTALL_PREFIX=OFF to our CMake flags

[pipcet]

TL;DR Can we apply this patch to a test system and see whether it makes things better or worse?

Here's the issue:

1. start with a fresh docker container; note that it doesn't (and shouldn't) contain a "ccache" binary in the path. (I used sha256:25e51469574b174a940bc7970cba71d8f96ce228c8ddf7a60aedd61f32c87c68)
2. ./build-package.sh -I ccache
3. ./build-package.sh -I swiftshader
4. /bin/sh: 1: ccache: not found

The build process assumes
there's a "ccache" binary in the path, even though the only copy of
"ccache" is that in the /data directory. The problem is that CMake's
find_program searches the install prefix unless
CMAKE_FIND_USE_INSTALL_PREFIX is set, even though
CMAKE_FIND_ROOT_PATH_MODE_PROGRAM=NEVER is also set and affects the
same directory.

Problems:

1. CMake doesn't document whether find_program is ever validly used
   to search for target executables rather than host executables. If
   that is the case, this change might break other packages.
2. swiftshader is broken. Its CMake code looks like this:

# Use CCache if available
find_program(CCACHE_FOUND ccache)
if(CCACHE_FOUND)
    message(STATUS "Using ccache")
    set_property(GLOBAL PROPERTY RULE_LAUNCH_COMPILE ccache)
    set_property(GLOBAL PROPERTY RULE_LAUNCH_LINK ccache)
endif()

Obviously, it's perfectly possible for ccache to be a findable program
but not reside on the PATH, but the code assumes it's always in the
cache.

(It's generally a bad idea to automatically enable ccache without an
explicit flag, but that's a swiftshader problem, and we have to live
with the broken CMakeLists.txt provided by the package maintainer)

This popped up while working on #22305, but is unrelated.

[TomJo2000]

That makes sense to me.
We wouldn't want to use programs from the /data/* directory as they are cross-compiled for Termux.

We may be implicitly relying on that for some packages but that is purely speculation on my part.

[Biswa96]

Could you please clarify whether this change might create any issues with the on-device package build?

[pipcet]

Could you please clarify whether this change might create any issues with the on-device package build?

Good point. Maybe we should move this change to the section:

 	if [ "$TERMUX_ON_DEVICE_BUILD" = "false" ]; then

@TomJo2000 , does that make sense to you? I don't know anything about on-device termux builds...

[TomJo2000]

Right I think that's what was nagging at the back of my mind.
During on-device builds those are valid for CMake to pick.