scripts: default to using .go-version's version for tests and builds

Additionally, provide ability to opt-out of the .go-version and use a
custom one via env vars: FORCE_HOST_GO and GO_VERSION.

Signed-off-by: Madhav Jivrajani <madhav.jiv@gmail.com>

scripts/test_lib.sh

@@ -384,3 +384,27 @@ function git_assert_branch_in_sync {
     log_warning "Cannot verify consistency with the origin, as git is on detached branch."
   fi
 }
+
+# The version present in the .go-verion is the default version that test and build scripts will use.
+# However, it is possible to control the version that should be used with the help of env vars:
+# - FORCE_HOST_GO: if set to a non-empty value, use the version of go installed in system's $PATH.
+# - GO_VERSION: desired version of go to be used, might differ from what is present in .go-version.
+#               If empty, the value defaults to the version in .go-version.
+function determine_go_version {
+  # Borrowing from how Kubernetes does this:
+  #  https://github.com/kubernetes/kubernetes/blob/17854f0e0a153b06f9d0db096e2cd8ab2fa89c11/hack/lib/golang.sh#L510-L520
+  #
+  # default GO_VERSION to content of .go-version
+  GO_VERSION="${GO_VERSION:-"$(cat "${ETCD_ROOT_DIR}/.go-version")"}"
+  if [ "${GOTOOLCHAIN:-auto}" != 'auto' ]; then
+    # no-op, just respect GOTOOLCHAIN
+    :
+  elif [ -n "${FORCE_HOST_GO:-}" ]; then
+    export GOTOOLCHAIN='local'
+  else
+    GOTOOLCHAIN="go${GO_VERSION}"
+    export GOTOOLCHAIN
+  fi
+}
+
+determine_go_version