STARPU_PERF_MODEL_DIR may be ignored if the host has already been calibrated in .starpu before

We have some unexpected behavior (probably since commit 86d0e7b8). We set STARPU_PERF_MODEL_DIR to toto, but the files are generated in $HOME/.starpu
If .starpu/sampling/bus/hostname.config already exists and the file $STARPU_PERF_MODEL_DIR/bus/hostname.config doesn't, the STARPU_PERF_MODEL_DIR is ignored and the result is stored in $HOME/.starpu

$ rm -rf .starpu toto
$ STARPU_PERF_MODEL_DIR=toto starpu_calibrate_bus
[starpu][l0][check_bus_config_file] No performance model for the bus, calibrating...
[starpu][l0][benchmark_all_memory_nodes] NUMA 0 -> 1...
[starpu][l0][benchmark_all_memory_nodes] NUMA 1 -> 0...
[starpu][l0][check_bus_config_file] ... done
$ find toto
toto
toto/bus
toto/bus/l0.affinity
toto/bus/l0.latency
toto/bus/l0.bandwidth
toto/bus/l0.config
toto/bus/l0.platform.xml
toto/bus/l0.platform.v4.xml
$ rm -rf toto
$ starpu_calibrate_bus
[starpu][l0][check_bus_config_file] No performance model for the bus, calibrating...
[starpu][l0][benchmark_all_memory_nodes] NUMA 0 -> 1...
[starpu][l0][benchmark_all_memory_nodes] NUMA 1 -> 0...
[starpu][l0][check_bus_config_file] ... done
$ find .starpu
.starpu
.starpu/sampling
.starpu/sampling/bus
.starpu/sampling/bus/l0.affinity
.starpu/sampling/bus/l0.latency
.starpu/sampling/bus/l0.bandwidth
.starpu/sampling/bus/l0.config
.starpu/sampling/bus/l0.platform.xml
.starpu/sampling/bus/l0.platform.v4.xml
$ STARPU_PERF_MODEL_DIR=toto starpu_calibrate_bus
[starpu][l0][benchmark_all_memory_nodes] NUMA 0 -> 1...
[starpu][l0][benchmark_all_memory_nodes] NUMA 1 -> 0...
$ find toto
find: ‘toto’: No such file or directory