Use bash explicitly to run build-protoc.sh to make it work on Windows w/ MSYS

protoc-artifacts/build.gradle

diff --git a/protoc-artifacts/build.gradle b/protoc-artifacts/build.gradle
index b95d00c..de93cb0 100644
--- a/protoc-artifacts/build.gradle
+++ b/protoc-artifacts/build.gradle
@@ -29,7 +29,7 @@
 def artifactFile = 'target/protoc.exe' as File
 
 task buildProtoc(type: Exec) {
-  commandLine './build-protoc.sh'
+  commandLine 'bash', 'build-protoc.sh'
 }
 
 task prepareArtifact(type: Copy, dependsOn: buildProtoc) {