Address new shellcheck nits

Also adopt new Ubuntu and actions/checkout.

Follows whatwg/html#8398.

Author: annevk

.github/workflows/build.yaml

@@ -9,12 +9,12 @@ on:
 jobs:
   build:
     name: Build
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-22.04
     env:
       IMAGE_NAME: whatwg/html-build
     steps:
     - name: Checkout whatwg/html-build
-      uses: actions/checkout@v2
+      uses: actions/checkout@v3
       with:
         fetch-depth: 0
     - name: Shellcheck
@@ -24,7 +24,7 @@ jobs:
     - name: Docker build
       run: ci-build/docker-build.sh
     - name: Checkout whatwg/html
-      uses: actions/checkout@v2
+      uses: actions/checkout@v3
       with:
         repository: whatwg/html
         path: html

build.sh

@@ -403,7 +403,7 @@ function relativePath {
   local commonPart=$source
   local result=""
 
-  while [[ "${target#$commonPart}" == "${target}" ]]; do
+  while [[ "${target#"$commonPart"}" == "${target}" ]]; do
     # no match, means that candidate common part is not correct
     # go up one level (reduce common part)
     commonPart=$(dirname "$commonPart")
@@ -422,7 +422,7 @@ function relativePath {
 
   # since we now have identified the common part,
   # compute the non-common part
-  local forwardPart="${target#$commonPart}"
+  local forwardPart="${target#"$commonPart"}"
 
   # and now stick all parts together
   if [[ $result != "" ]] && [[ $forwardPart != "" ]]; then