nf-core lint throws exception when --dir is used
Running `nf-core lint` throws the following exception when using the `--dir` option

## Steps to reproduce

```bash
$ nf-core lint --dir nf-core-testpipeline

                                          ,--./,-.
          ___     __   __   __   ___     /,-._.--~\\
    |\\ | |__  __ /  ` /  \\ |__) |__         }  {
    | \\| |       \\__, \\__/ |  \\ |___     \\`-._,-`-,
                                          `._,._,'

    nf-core/tools version 2.2.dev0



ERROR    Could not determine repository type of 'nf-core-testpipeline'                                                                                       __main__.py:345
```
If the lint is run from the pipeline folder it works.

## Expected behaviour

<!-- A clear and concise description of what you expected to happen. -->

## System

- OS: macOS
- Version of nf-core/tools: 2.2.dev0
- Python version: Tested on 3.7 and 3.8

## Additional context

The reason behind the error is that in #1322 the `os.path.join` was removed from the code.

diff --git a/nf_core/modules/module_utils.py b/nf_core/modules/module_utils.py
index 6ef675bf..ce8a8027 100644
--- a/nf_core/modules/module_utils.py
+++ b/nf_core/modules/module_utils.py
@@ -344,8 +344,8 @@ def get_repo_type(dir):
         raise LookupError("Could not find directory: {}".format(dir))
 
     # Determine repository type
-    if os.path.exists("README.md"):
-        with open("README.md") as fh:
+    if os.path.exists(os.path.join(dir, "README.md")):
+        with open(os.path.join(dir, "README.md")) as fh:
             if fh.readline().rstrip().startswith("# ![nf-core/modules]"):
                 return "modules"
             else:
['nf_core/modules/module_utils.py']