Fix exception in 'test cases/failing/55 wrong shared crate type' when rustc is missing

Fix exception handling of missing rustc, by making it look like the other
compiler detectors

Traceback (most recent call last):
  File "/wip/meson/mesonbuild/environment.py", line 699, in detect_rust_compiler
    p, out = Popen_safe(compiler + ['--version'])[0:2]
[...]
FileNotFoundError: [Errno 2] No such file or directory: 'rustc': 'rustc'

During handling of the above exception, another exception occurred:

[...]
  File "/wip/meson/mesonbuild/environment.py", line 701, in detect_rust_compiler
    popen_exceptions[compiler] = e
TypeError: unhashable type: 'list'

mesonbuild/environment.py

@@ -695,10 +695,11 @@ class Environment:
         for compiler in compilers:
             if isinstance(compiler, str):
                 compiler = [compiler]
+            arg = ['--version']
             try:
-                p, out = Popen_safe(compiler + ['--version'])[0:2]
+                p, out = Popen_safe(compiler + arg)[0:2]
             except OSError as e:
-                popen_exceptions[compiler] = e
+                popen_exceptions[' '.join(compiler + arg)] = e
                 continue
 
             version = search_version(out)