pip_install_wheel: don't lazy assign PIPINSTALLARGS

If we expect users to extend this we should use =, as otherwise a recipe
that does += will replace the default value.

(From OE-Core rev: fdaf5e0027a52e44f2def0ef240134763660aa00)

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/pip_install_wheel.bbclass

@@ -10,7 +10,7 @@ PIP_INSTALL_PACKAGE ?= "${@guess_pip_install_package_name(d)}"
 PIP_INSTALL_DIST_PATH ?= "${@d.getVar('SETUPTOOLS_SETUP_PATH') or d.getVar('B')}/dist"
 PYPA_WHEEL ??= "${PIP_INSTALL_DIST_PATH}/${PIP_INSTALL_PACKAGE}-*-*.whl"
 
-PIP_INSTALL_ARGS ?= "\
+PIP_INSTALL_ARGS = "\
     -vvvv \
     --ignore-installed \
     --no-cache \