ci(buildkite): run builds on PRs opened from forks as well

Previously, the buildkite jobs wouldn't launch for PRs that were opened
on forks, because the `smooth-checkout` plugin would not be able to find
a valid refspec[^1]. This fixes this up for the most general cases, i.e.,
when a build is kicked off because a PR is opened or more commits are
added on to the PR, and is not expected to work when builds are
explicitly triggered (through the Web UI) for the `HEAD` commit.

[^1]: https://buildkite.com/julialang/scimldocs/builds/3324#018cb654-71a9-4ce3-9af3-d7e21d044397/101-119

.buildkite/0_webui.yml

@@ -1,6 +1,12 @@
 steps:
   - label: ":rocket: Launch Pipeline!"
     plugins:
+      - hasura/smooth-checkout#v4.4.1:
+          repos:
+            - config:
+                - url: "https://github.com/SciML/SciMLDocs.git"
+                  ref: "${BUILDKITE_COMMIT}"
+                  clone_flags: "--depth=1"
       - staticfloat/cryptic#v2:
           signed_pipelines:
             - pipeline: .buildkite/aggregate.yml

.buildkite/aggregate.yml

@@ -9,6 +9,7 @@ steps:
           repos:
             - config:
                 - url: "https://github.com/SciML/SciMLDocs.git"
+                  ref: "${BUILDKITE_COMMIT}"
                   clone_flags: "--depth=1"
 
       - staticfloat/cryptic#v2:

.buildkite/aggregate.yml.signature

@@ -1 +1 @@
-Salted__��CeNFm��A{2KJ�5���d0o\�D@D{]$�%���'i�X�j'|G�$�����9V���A�U]��7&y�0��q\{�W��k�7
+Salted__f��Ǘ{/���qO�w����{�bx'0P���SM�����}K��f*�-7�,�I�QO|A&V���ňzQ��I�8RG�#

.buildkite/documentation.yml

@@ -7,6 +7,7 @@ steps:
           repos:
             - config:
                 - url: "https://github.com/SciML/SciMLDocs.git"
+                  ref: "${BUILDKITE_COMMIT}"
                   clone_flags: "--depth=1"
 
     command: |