utils: fallback to env var when detecting shell (#2147)

Resolves: #2115

poetry/utils/shell.py

@@ -9,6 +9,7 @@
 from shellingham import detect_shell
 
 from ._compat import WINDOWS
+from ._compat import Path
 from .env import VirtualEnv
 
 
@@ -42,7 +43,17 @@ def get(cls):  # type: () -> Shell
         try:
             name, path = detect_shell(os.getpid())
         except (RuntimeError, ShellDetectionFailure):
-            raise RuntimeError("Unable to detect the current shell.")
+            shell = None
+
+            if os.name == "posix":
+                shell = os.environ.get("SHELL")
+            elif os.name == "nt":
+                shell = os.environ.get("COMSPEC")
+
+            if not shell:
+                raise RuntimeError("Unable to detect the current shell.")
+
+            name, path = Path(shell).stem, shell
 
         cls._shell = cls(name, path)