This PR allows the user to specify a path to a custom cert file (or directory) in
Spack's config:
```yaml
# This is where custom certs for proxy/firewall are stored.
# It can be a path or environment variable. To match ssl env configuration
# the default is the environment variable SSL_CERT_FILE
ssl_certs: $SSL_CERT_FILE
```
`config:ssl_certs` can be a path to a file or a directory, or it can be and environment
variable that resolves to one of those. When it posts to something valid, Spack will
update the ssl context to include custom certs, and fetching via `urllib` and `curl`
will trust the provided certs.
This should resolve many issues with fetching behind corporate firewalls.
---------
Co-authored-by: psakievich <psakievich@users.noreply.github.com>
Co-authored-by: Alec Scott <alec@bcs.sh>
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