Deps: Specify prefix path for Qt build

[TheLastRar]

Description of Changes

Specify -DCMAKE_PREFIX_PATH when building Qt on Linux

Rationale behind Changes

Local installs of Qt could break the build
Other platforms specify this already

Suggested Testing Steps

Install an older version of Qt on the system (like 6.8)
Run the deps script and see if the build completes

Did you use AI to help find, test, or implement this issue or feature?

No

[TellowKrinkle]

Might want to use a list of common arguments like the macOS build

pcsx2/.github/workflows/scripts/macos/build-dependencies.sh

Lines 50 to 57 in 33ab6fd

	CMAKE_COMMON=(
	-DCMAKE_BUILD_TYPE=Release
	-DCMAKE_SHARED_LINKER_FLAGS="-dead_strip -dead_strip_dylibs"
	-DCMAKE_PREFIX_PATH="$INSTALLDIR"
	-DCMAKE_INSTALL_PREFIX="$INSTALLDIR"
	-DCMAKE_OSX_ARCHITECTURES="x86_64"
	-DCMAKE_INSTALL_NAME_DIR='$<INSTALL_PREFIX>/lib'
	)