Upgrade bazelisk from v1.6.1 to bug-fixing 1.7.3

This release holds fixes and a few changes since; cf. [1].

[1] https://github.com/bazelbuild/bazelisk/compare/v1.6.1...v1.7.3

Change-Id: I751f05a7eef812f5e2208e91ededbeb32ff36f7b
diff --git a/jenkins-docker/slave-bazel-nocache/Makefile b/jenkins-docker/slave-bazel-nocache/Makefile
index b939fb0..ff983a7 100644
--- a/jenkins-docker/slave-bazel-nocache/Makefile
+++ b/jenkins-docker/slave-bazel-nocache/Makefile
@@ -4,7 +4,7 @@
 IMAGE=gerritforge/gerrit-ci-slave-bazel:debian-buster-nocache
 BAZEL_VER=3.5.1
 BUILDIFIER_VER=3.5.0
-BAZELISK_VER=1.6.1
+BAZELISK_VER=1.7.3
 NODE_VER=10
 WCT_VER=6.8.0