ppci issues in daily install

```
ERROR OUTPUT:
cc65 V2.17 - Git 8e75906
gcc version 1.27
cc65 V2.17 - Git f95481f
/bin/rm: cannot remove '/tmp/tcc-PIUGWS4NI5/tcc_startup.h': No such file or directory
/bin/rm: cannot remove '/tmp/tcc-PIUGWS4NI5': No such file or directory
Traceback (most recent call last):
File "/usr/lib/python3.8/runpy.py", line 193, in _run_module_as_main
return _run_code(code, main_globals, None,
File "/usr/lib/python3.8/runpy.py", line 86, in _run_code
exec(code, run_globals)
File "/opt/compiler-explorer/ppci-0.5.5/ppci/__main__.py", line 3, in
from ppci.cli import build
File "/opt/compiler-explorer/ppci-0.5.5/ppci/cli/build.py", line 8, in
from .base import base_parser, LogSetup
File "/opt/compiler-explorer/ppci-0.5.5/ppci/cli/base.py", line 18, in
platform.platform())
File "/usr/lib/python3.8/platform.py", line 1205, in platform
libcname, libcversion = libc_ver(sys.executable)
File "/usr/lib/python3.8/platform.py", line 193, in libc_ver
with open(executable, 'rb') as f:
IsADirectoryError: [Errno 21] Is a directory: '/opt/compiler-explorer'
Traceback (most recent call last):
File "/usr/lib/python3.8/runpy.py", line 193, in _run_module_as_main
return _run_code(code, main_globals, None,
File "/usr/lib/python3.8/runpy.py", line 86, in _run_code
exec(code, run_globals)
File "/opt/compiler-explorer/ppci-0.5.5/ppci/__main__.py", line 3, in
from ppci.cli import build
File "/opt/compiler-explorer/ppci-0.5.5/ppci/cli/build.py", line 8, in
from .base import base_parser, LogSetup
File "/opt/compiler-explorer/ppci-0.5.5/ppci/cli/base.py", line 18, in
platform.platform())
File "/usr/lib/python3.8/platform.py", line 1205, in platform
libcname, libcversion = libc_ver(sys.executable)
File "/usr/lib/python3.8/platform.py", line 193, in libc_ver
with open(executable, 'rb') as f:
IsADirectoryError: [Errno 21] Is a directory: '/opt/compiler-explorer'
```

Start by reproducing the daily install failure and read /opt/compiler-explorer/ppci-0.5.5/ppci/__main__.py and ppci/cli/base.py around the platform.platform() call. Trace why sys.executable is treated as the /opt/compiler-explorer directory; done means the install completes without the reported IsADirectoryError.

Written by the indexing model from the issue text.