build: fix snapshot script to be executable

[yoshi-automation]

This PR was generated using Autosynth. 🌈

Here's the log from Synthtool:

synthtool > Executing /tmpfs/src/git/autosynth/working_repo/synth.py.
.github/ISSUE_TEMPLATE/bug_report.md
.github/ISSUE_TEMPLATE/feature_request.md
.github/ISSUE_TEMPLATE/support_request.md
.github/PULL_REQUEST_TEMPLATE.md
.kokoro/build.bat
.kokoro/build.sh
.kokoro/common.cfg
.kokoro/continuous/common.cfg
.kokoro/continuous/dependencies.cfg
.kokoro/continuous/integration.cfg
.kokoro/continuous/java11.cfg
.kokoro/continuous/java7.cfg
.kokoro/continuous/java8-osx.cfg
.kokoro/continuous/java8-win.cfg
.kokoro/continuous/java8.cfg
.kokoro/continuous/lint.cfg
.kokoro/continuous/propose_release.cfg
.kokoro/continuous/propose_release.sh
.kokoro/dependencies.sh
.kokoro/linkage-monitor.sh
.kokoro/nightly/common.cfg
.kokoro/nightly/dependencies.cfg
.kokoro/nightly/integration.cfg
.kokoro/nightly/java11.cfg
.kokoro/nightly/java7.cfg
.kokoro/nightly/java8-osx.cfg
.kokoro/nightly/java8-win.cfg
.kokoro/nightly/java8.cfg
.kokoro/nightly/lint.cfg
.kokoro/presubmit/common.cfg
.kokoro/presubmit/dependencies.cfg
.kokoro/presubmit/integration.cfg
.kokoro/presubmit/java11.cfg
.kokoro/presubmit/java7.cfg
.kokoro/presubmit/java8-osx.cfg
.kokoro/presubmit/java8-win.cfg
.kokoro/presubmit/java8.cfg
.kokoro/presubmit/linkage-monitor.cfg
.kokoro/presubmit/lint.cfg
.kokoro/release/bump_snapshot.cfg
.kokoro/release/bump_snapshot.sh
.kokoro/release/common.cfg
.kokoro/release/common.sh
.kokoro/release/drop.cfg
.kokoro/release/drop.sh
.kokoro/release/promote.cfg
.kokoro/release/promote.sh
.kokoro/release/publish_javadoc.cfg
.kokoro/release/publish_javadoc.sh
.kokoro/release/snapshot.cfg
.kokoro/release/snapshot.sh
.kokoro/release/stage.cfg
.kokoro/release/stage.sh
.kokoro/trampoline.sh
CODE_OF_CONDUCT.md
CONTRIBUTING.md
LICENSE
README.md
codecov.yaml
java.header
license-checks.xml
renovate.json
synthtool > Cleaned up 2 temporary directories.
synthtool > Wrote metadata to synth.metadata.