ce_install github with prefix fails on first run

With `target_prefix: v` in `clone_branch`, the first time you run you get:

```
2020-10-19 21:02:27,260 lib.installation INFO compilers/go/nightly/golang trunk: Most recent go-trunk is 20201019
Installing libraries/cuda/nightly/nvtx dev
2020-10-19 21:02:30,000 lib.installation INFO libraries/cuda/nightly/nvtx dev installed OK
Installing libraries/cuda/nvtx 3.0.1
error: pathspec '3.0.1' did not match any file(s) known to git
Traceback (most recent call last):
File "", line 1, in
File "/home/ubuntu/infra/bin/lib/ce_install.py", line 178, in main
if installable.install():
File "/home/ubuntu/infra/bin/lib/installation.py", line 512, in install
self.stage()
File "/home/ubuntu/infra/bin/lib/installation.py", line 491, in stage
self.clone_branch()
File "/home/ubuntu/infra/bin/lib/installation.py", line 451, in clone_branch
subprocess.check_call(['git', '-C', dest, 'checkout', '-q', self.target_name],
File "/usr/lib/python3.8/subprocess.py", line 364, in check_call
raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['git', '-C', '/opt/compiler-explorer/libs/nvtx/v3.0.1', 'checkout', '-q', '3.0.1']' returned non-zero exit status 1.
```

The result is a directory called `v3.0.1` :

```
$ ls /opt/compiler-explorer/libs/nvtx/
v3.0.1/
```

the second run completes without issue.

```
Installing libraries/cuda/nvtx 3.0.1
2020-10-19 21:04:10,582 lib.installation INFO libraries/cuda/nvtx 3.0.1 is already installed, skipping
```

I was imagining the prefix would apply to the `tag` checked out, not the directory on disk? Maybe I misunderstand; that is I expected the direcotyr to be `{name}` and the tag `v{name}` but obviously I can rename. Either way, the first run doesn't complete successfully, perhaps due to confusion elsewhere.