testsuite: fix on systems with flux-accounting already installed

Problem: When flux-accounting is already installed on a system, the
sharness tests pull in the installed version of flux-accounting Python
modules instead of the test version. This occurs because sharness
prepends the source tree Python path to PYTHONPATH, which is then
subverted by flux-core's `py-runner.py`, which prepends the system
path in front of that before executing Python subcommands like the
accounting service etc.

Use FLUX_PYTHONPATH_PREPEND instead of PYTHONPATH, which is preserved
by the py-runner.py wrapper.

t/sharness.d/flux-accounting.sh

@@ -12,8 +12,8 @@ prepend_colon_separated() {
 SRC_DIR=${SHARNESS_TEST_SRCDIR}/..
 
 prepend_colon_separated FLUX_EXEC_PATH_PREPEND ${SRC_DIR}/src/cmd
-prepend_colon_separated PYTHONPATH ${SRC_DIR}/src/bindings/python
+prepend_colon_separated FLUX_PYTHONPATH_PREPEND ${SRC_DIR}/src/bindings/python
 
-export FLUX_EXEC_PATH_PREPEND PYTHONPATH
+export FLUX_EXEC_PATH_PREPEND FLUX_PYTHONPATH_PREPEND
 
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