chainer-head test does not use head versions as expected

[kmaehashi]

https://jenkins.preferred.jp/job/chainer/job/daily_master_head/TEST=cupy-head,label=mn1-p100/595/console

+ python -c 'import cupy; cupy.show_config()'
OS                           : Linux-4.4.0-210-generic-x86_64-with-debian-stretch-sid
Python Version               : 3.6.6
CuPy Version                 : 10.0.0a1
CuPy Platform                : NVIDIA CUDA
NumPy Version                : 1.19.5
SciPy Version                : 1.5.4
Cython Build Version         : 0.29.23
Cython Runtime Version       : 0.29.23
CUDA Root                    : /usr/local/cuda
nvcc PATH                    : ccache nvcc --compiler-bindir g++-6
CUDA Build Version           : 10010
CUDA Driver Version          : 11030
CUDA Runtime Version         : 10010
cuBLAS Version               : (available)
cuFFT Version                : 10101
cuRAND Version               : 10101
cuSOLVER Version             : (10, 2, 0)
cuSPARSE Version             : (available)
NVRTC Version                : (10, 1)
Thrust Version               : 100906
CUB Build Version            : 100800
Jitify Build Version         : 60e9e72
cuDNN Build Version          : 7605
cuDNN Version                : 7605
NCCL Build Version           : 2402
NCCL Runtime Version         : 2402
cuTENSOR Version             : None
cuSPARSELt Build Version     : None
Device 0 Name                : NVIDIA Tesla P100-PCIE-12GB
Device 0 Compute Capability  : 60
Device 0 PCI Bus ID          : 0000:0B:00.0
Device 1 Name                : NVIDIA Tesla P100-PCIE-12GB
Device 1 Compute Capability  : 60
Device 1 PCI Bus ID          : 0000:0C:00.0
Device 2 Name                : NVIDIA Tesla P100-PCIE-12GB
Device 2 Compute Capability  : 60
Device 2 PCI Bus ID          : 0000:0D:00.0
Device 3 Name                : NVIDIA Tesla P100-PCIE-12GB
Device 3 Compute Capability  : 60
Device 3 PCI Bus ID          : 0000:0E:00.0
Device 4 Name                : NVIDIA Tesla P100-PCIE-12GB
Device 4 Compute Capability  : 60
Device 4 PCI Bus ID          : 0000:04:00.0
Device 5 Name                : NVIDIA Tesla P100-PCIE-12GB
Device 5 Compute Capability  : 60
Device 5 PCI Bus ID          : 0000:05:00.0
Device 6 Name                : NVIDIA Tesla P100-PCIE-12GB
Device 6 Compute Capability  : 60
Device 6 PCI Bus ID          : 0000:06:00.0
Device 7 Name                : NVIDIA Tesla P100-PCIE-12GB
Device 7 Compute Capability  : 60
Device 7 PCI Bus ID          : 0000:07:00.0