Don't require $GITHUB_TOKEN to build locally

Mdbook currently gives a hard error if it's not set in the environment:
```
[2023-03-25T22:52:13Z ERROR mdbook_linkcheck::config] Unable to interpolate "authorization: Bearer $GITHUB_TOKEN" because Failed to retrieve `GITHUB_TOKEN` env var: environment variable not found
```

Dynamically add it in `linkcheck.sh` to avoid requiring it when we don't
need it.

Author: jyn514

README.md

@@ -49,10 +49,10 @@ To build a local static HTML site, install [`mdbook`](https://github.com/rust-la
 and execute the following command in the root of the repository:
 
 ```
-> mdbook build
+> mdbook build --open
 ```
 
-The build files are found in the `book` directory.
+The build files are found in the `book/html` directory.
 
 ### Link Validations
 

book.toml

@@ -43,9 +43,6 @@ exclude = [
 cache-timeout = 86400
 warning-policy = "error"
 
-[output.linkcheck.http-headers]
-'github\.com' = ["Authorization: Bearer $GITHUB_TOKEN"]
-
 [output.html.redirect]
 "/compiletest.html" = "tests/compiletest.html"
 "/diagnostics/sessiondiagnostic.html" = "diagnostic-structs.html"

ci/linkcheck.sh

@@ -3,9 +3,15 @@
 set -e
 set -o pipefail
 
+set_github_token() {
+  echo "[output.linkcheck.http-headers]
+'github\.com' = [\"Authorization: Bearer \$GITHUB_TOKEN\"]" >> $(git rev-parse --show-toplevel)/book.toml
+}
+
 # https://docs.github.com/en/actions/reference/environment-variables
 if [ "$GITHUB_EVENT_NAME" = "schedule" ] ; then # running in scheduled job
   FLAGS=""
+  set_github_token
 
   echo "Doing full link check."
   set -x
@@ -17,6 +23,7 @@ elif [ "$GITHUB_EVENT_NAME" = "pull_request" ] ; then # running in PR CI build
 
   CHANGED_FILES=$(git diff --name-only $BASE_SHA... | tr '\n' ' ')
   FLAGS="--no-cache -f $CHANGED_FILES"
+  set_github_token
 
   echo "Checking files changed since $BASE_SHA: $CHANGED_FILES"
   set -x