Reduce logging at startup

There are enough unset env variables that I've stopped reading them all,
and this has just become noise.

Before, useful information would be drowned in debug logging:

```
2021/04/18 13:56:21 [DEBUG] docs_rs::config: optional configuration variable DOCSRS_BUILD_ATTEMPTS is not set
2021/04/18 13:56:21 [DEBUG] docs_rs::config: optional configuration variable REGISTRY_INDEX_PATH is not set
2021/04/18 13:56:21 [DEBUG] docs_rs::config: optional configuration variable REGISTRY_URL is not set
2021/04/18 13:56:21 [DEBUG] docs_rs::config: optional configuration variable DOCSRS_MAX_POOL_SIZE is not set
2021/04/18 13:56:21 [DEBUG] docs_rs::config: optional configuration variable DOCSRS_MIN_POOL_IDLE is not set
2021/04/18 13:56:21 [DEBUG] docs_rs::config: optional configuration variable DOCSRS_S3_BUCKET is not set
2021/04/18 13:56:21 [DEBUG] docs_rs::config: optional configuration variable S3_REGION is not set
2021/04/18 13:56:21 [DEBUG] docs_rs::config: optional configuration variable S3_ENDPOINT is not set
2021/04/18 13:56:21 [DEBUG] docs_rs::config: optional configuration variable DOCSRS_GITHUB_UPDATER_MIN_RATE_LIMIT is not set
2021/04/18 13:56:21 [DEBUG] docs_rs::config: optional configuration variable DOCSRS_GITLAB_ACCESSTOKEN is not set
2021/04/18 13:56:21 [DEBUG] docs_rs::config: optional configuration variable DOCSRS_MAX_FILE_SIZE is not set
2021/04/18 13:56:21 [DEBUG] docs_rs::config: optional configuration variable DOCSRS_MAX_FILE_SIZE_HTML is not set
2021/04/18 13:56:21 [DEBUG] docs_rs::config: optional configuration variable DOCSRS_MAX_PARSE_MEMORY is not set
2021/04/18 13:56:21 [DEBUG] docs_rs::config: optional configuration variable DOCSRS_REGISTRY_GC_INTERVAL is not set
2021/04/18 13:56:21 [DEBUG] docs_rs::config: optional configuration variable DOCSRS_RANDOM_CRATE_SEARCH_VIEW_SIZE is not set
2021/04/18 13:56:21 [DEBUG] docs_rs::config: optional configuration variable DOCSRS_CSP_REPORT_ONLY is not set
2021/04/18 13:56:21 [DEBUG] docs_rs::config: optional configuration variable DOCS_RS_DOCKER is not set
2021/04/18 13:56:21 [DEBUG] docs_rs::config: optional configuration variable DOCS_RS_LOCAL_DOCKER_IMAGE is not set
2021/04/18 13:56:21 [DEBUG] docs_rs::config: optional configuration variable DOCSRS_INCLUDE_DEFAULT_TARGETS is not set
2021/04/18 13:56:21 [DEBUG] docs_rs::config: optional configuration variable DOCSRS_DISABLE_MEMORY_LIMIT is not set
2021/04/18 09:57:17 [WARN] docs_rs::repositories::github: did not collect `github.com` stats as no token was provided
2021/04/18 09:57:17 [WARN] docs_rs::repositories::gitlab: will try to retrieve `gitlab.com` stats without token since none was provided
2021/04/18 09:57:17 [WARN] docs_rs::repositories::gitlab: will try to retrieve `gitlab.freedesktop.org` stats without token since none was provided
2021/04/18 09:57:17 [INFO] docs_rs::web: Running docs.rs web server on http://0.0.0.0:3000
```

Now, only useful info is shown:

```
2021/04/18 09:57:17 [WARN] docs_rs::repositories::github: did not collect `github.com` stats as no token was provided
2021/04/18 09:57:17 [WARN] docs_rs::repositories::gitlab: will try to retrieve `gitlab.com` stats without token since none was provided
2021/04/18 09:57:17 [WARN] docs_rs::repositories::gitlab: will try to retrieve `gitlab.freedesktop.org` stats without token since none was provided
2021/04/18 09:57:17 [INFO] docs_rs::web: Running docs.rs web server on http://0.0.0.0:3000
```

Author: jyn514

src/config.rs

@@ -144,7 +144,7 @@ where
             .map(Some)
             .with_context(|_| format!("failed to parse configuration variable {}", var))?),
         Err(VarError::NotPresent) => {
-            log::debug!("optional configuration variable {} is not set", var);
+            log::trace!("optional configuration variable {} is not set", var);
             Ok(None)
         }
         Err(VarError::NotUnicode(_)) => bail!("configuration variable {} is not UTF-8", var),