[simdutf] make cifuzz work better for pull requests (#12293)

I recently made changes to the fuzzer build script in the simdutf repo.
The CIfuzz job failed.

The reason is that CIfuzz checks out the default branch when building
the docker image.
The current dockerfile copied the build script into the docker image.

This means the build script is used from the default branch, while doing
CIfuzz on the PR branch. This broke when I made changes to the build
script.

With the changes in this PR, the build script from the checked out code
(the PR to run CIfuzz on) is executed instead. That should make it more
robust, and make it possible to work with improvements in the build
script without having to merge first.

Ping @lemire

projects/simdutf/Dockerfile

@@ -17,5 +17,5 @@
 FROM gcr.io/oss-fuzz-base/base-builder
 RUN apt-get update && apt-get install -y make cmake
 RUN git clone --depth 1 https://github.com/simdutf/simdutf simdutf
-RUN cp simdutf/fuzz/build.sh $SRC/
-WORKDIR simdutf
+WORKDIR simdutf
+COPY build.sh $SRC/

projects/simdutf/build.sh

@@ -0,0 +1,18 @@
+#!/bin/bash -eu
+# Copyright 2024 Google Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+################################################################################
+
+fuzz/build.sh