fix: reuse local Java before downloading runtime

scripts/test-install-fixture.sh

@@ -23,6 +23,7 @@ fail() {
   sed -n '/^is_semver()/,/^}/p' "$ROOT/deploy/install.sh"
   sed -n '/^write_env_file_preserving_unknown()/,/^}/p' "$ROOT/deploy/install.sh"
   sed -n '/^azul_arch()/,/^}/p' "$ROOT/deploy/install.sh"
+  sed -n '/^system_java_home()/,/^}/p' "$ROOT/deploy/install.sh"
   sed -n '/^sha256_file()/,/^}/p' "$ROOT/deploy/install.sh"
   sed -n '/^prepare_java()/,/^}/p' "$ROOT/deploy/install.sh"
   sed -n '/^download_release_url()/,/^}/p' "$ROOT/deploy/install.sh"
@@ -157,8 +158,15 @@ download() {
   esac
 }
 ARCH_FIXTURE=i686
+export KAIDI_JAVA_HOME="$WORK/missing-java-home"
 prepare_java >/dev/null 2>&1
 [ -x "$JAVA_STAGED_DIR/bin/java" ] || fail 'verified i686 Java runtime was not staged'
+rm -rf "$JAVA_STAGED_DIR"
+export KAIDI_JAVA_HOME="$WORK/fake-jre"
+prepare_java >/dev/null 2>&1
+[ -L "$JAVA_STAGED_DIR" ] || fail 'existing Java 17 runtime was not reused'
+[ "$(readlink -f "$JAVA_STAGED_DIR")" = "$(readlink -f "$WORK/fake-jre")" ] \
+  || fail 'installer linked an unexpected local Java runtime'
 
 export RELEASE_API_URL=https://gitea.fixture.invalid/api/v1/repos/ERP-Team/kaidi/releases/latest
 RELEASE_TOKEN=fixture-read-only-token
@@ -225,6 +233,8 @@ grep -Fq 'download "$api" "$java_metadata"' "$ROOT/deploy/install.sh" \
 # shellcheck disable=SC2016 # Match literal installer source.
 grep -Fq 'java_sha256=$(jq -er' "$ROOT/deploy/install.sh" \
   || fail 'installer no longer obtains the Java runtime SHA-256'
+grep -Fq 'Using existing Java 17 runtime' "$ROOT/deploy/install.sh" \
+  || fail 'installer no longer reuses a local Java 17 runtime'
 # shellcheck disable=SC2016 # Match literal installer source.
 grep -Fq '[ "$actual_sha256" = "$java_sha256" ]' "$ROOT/deploy/install.sh" \
   || fail 'installer no longer verifies the Java runtime SHA-256'