sam build obscures pip ssl errors

### Description:

When running in an environment that requires you to pass a cert to pip, running sam build fails with an ssl error. There is no way to directly specify a cert file via a flag to the sam build command, the way you can when running pip directly. In many cases (e.g. when all your requirements are already available locally) the traceback you receive makes no mention of an ssl error.

The only way to specify a cert is to write a pip config file that sets the ssl file path, which is a bit cumbersome for a CI pipeline run.

### Steps to reproduce:

Run sam build in an environment that requires you to pass a cert file to pip

### Observed result:

```
56 2021-01-04 16:20:42,731 | Running PythonPipBuilder:ResolveDependencies
57 2021-01-04 16:20:42,756 | calling pip download -r /requirements.txt --dest /tmp/tmpv51uhu6u
58 2021-01-04 16:20:51,284 | PythonPipBuilder:ResolveDependencies failed
59 Traceback (most recent call last):
60 File "/usr/lib/python3.8/site-packages/aws_lambda_builders/workflows/python_pip/actions.py", line 39, in execute
61 package_builder.build_dependencies(
62 File "/usr/lib/python3.8/site-packages/aws_lambda_builders/workflows/python_pip/packager.py", line 137, in build_dependencies
63 self._dependency_builder.build_site_packages(requirements_path, artifacts_dir_path, scratch_dir_path)
64 File "/usr/lib/python3.8/site-packages/aws_lambda_builders/workflows/python_pip/packager.py", line 204, in build_site_packages
65 wheels, packages_without_wheels = self._download_dependencies(scratch_directory, requirements_filepath)
66 File "/usr/lib/python3.8/site-packages/aws_lambda_builders/workflows/python_pip/packager.py", line 228, in _download_dependencies
67 deps = self._download_all_dependencies(requirements_filename, directory)
68 File "/usr/lib/python3.8/site-packages/aws_lambda_builders/workflows/python_pip/packager.py", line 310, in _download_all_dependencies
69 self._pip.download_all_dependencies(requirements_filename, directory)
70 File "/usr/lib/python3.8/site-packages/aws_lambda_builders/workflows/python_pip/packager.py", line 599, in download_all_dependencies
71 raise NoSuchPackageError(str(package_name))
72 aws_lambda_builders.workflows.python_pip.packager.NoSuchPackageError: Could not satisfy the requirement: path
73 During handling of the above exception, another exception occurred:
74 Traceback (most recent call last):
75 File "/usr/lib/python3.8/site-packages/aws_lambda_builders/workflow.py", line 269, in run
76 action.execute()
77 File "/usr/lib/python3.8/site-packages/aws_lambda_builders/workflows/python_pip/actions.py", line 45, in execute
78 raise ActionFailedError(str(ex))
79 aws_lambda_builders.actions.ActionFailedError: Could not satisfy the requirement: path
80 Build Failed
81 2021-01-04 16:20:51,287 | Sending Telemetry: {'metrics': [{'commandRun': {'awsProfileProvided': False, 'debugFlagProvided': True, 'region': '', 'commandName': 'sam build', 'duration': 8931, 'exitReason': 'WorkflowFailedError', 'exitCode': 1, 'requestId': '84efccb7-1d4a-40a3-8cac-e107768315e4', 'installationId': '102a9a74-feb5-4f9f-aaac-429c61e7d91c', 'sessionId': 'af7ec9a7-63c9-45d3-9d3d-1f428159db2d', 'executionEnvironment': 'CLI', 'pyversion': '3.8.5', 'samcliVersion': '1.15.0'}}]}
82 2021-01-04 16:20:51,599 | Telemetry response: 200
83 Error: PythonPipBuilder:ResolveDependencies - Could not satisfy the requirement: path
```

### Expected result:

The error output clearly states that the build failed because a cert needs to be specified, and there is a more intuitive, documented way of specifying that cert.

### Additional environment details (Ex: Windows, Mac, Amazon Linux etc)

1. OS: amazon linux 2
2. `sam --version`: SAM CLI, version 1.15.0

`Add --debug flag to command you are running`