Strip quotes from environment variables when parsing from activate.sh (#277)

This fixes parsing of environment variables that are quoted in
`activate.sh`, such as

```
export NCCL_NET='AWS Libfabric'
```

Before this PR the above would be parsed to the value `'AWS Libfabric'`,
literally, including the quotes. The quotes would be removed by bash,
but when uenv sets environment variables the quotes are retained. In the
particular case of `NCCL_NET` the quoted value is not a valid name for a
plugin name, so NCCL fails to find the plugin.

Author: msimberg

stackinator/etc/envvars.py

@@ -399,7 +399,7 @@ def read_activation_script(filename: str, env: Optional[EnvVarSet] = None) -> En
                     continue
 
                 # rhs the value that is assigned to the environment variable
-                rhs = fields[1]
+                rhs = fields[1].lstrip("'").rstrip("'")
                 if name in list_variables:
                     fields = [f for f in rhs.split(":") if len(f.strip()) > 0]
                     # look for $name as one of the fields (only works for append or prepend)