Fix build.sh for macOs users (knative#883)

* Fix build.sh for macOs users

Fixes knative#882

* cleanup tempdir after installing

hack/build.sh

@@ -16,9 +16,6 @@
 
 set -o pipefail
 
-[[ ! -v REPO_ROOT_DIR ]] && REPO_ROOT_DIR="$(git rev-parse --show-toplevel)"
-readonly REPO_ROOT_DIR
-
 source_dirs="cmd pkg test lib"
 
 # Store for later
@@ -113,27 +110,16 @@ go_fmt() {
   find $(echo $source_dirs) -name "*.go" -print0 | xargs -0 gofmt -s -w
 }
 
-# Run a go tool, installing it first if necessary.
-# Parameters: $1 - tool package/dir for go get/install.
-#             $2 - tool to run.
-#             $3..$n - parameters passed to the tool.
+# Run a go tool, get it first if necessary.
 run_go_tool() {
   local tool=$2
   local install_failed=0
-  if [[ -z "$(which ${tool})" ]]; then
-    local action=get
-    [[ $1 =~ ^[\./].* ]] && action=install
-    # Avoid running `go get` from root dir of the repository, as it can change go.sum and go.mod files.
-    # See discussions in https://github.com/golang/go/issues/27643.
-    if [[ ${action} == "get" && $(pwd) == "${REPO_ROOT_DIR}" ]]; then
-      local temp_dir="$(mktemp -d)"
-      # Swallow the output as we are returning the stdout in the end.
-      pushd "${temp_dir}" > /dev/null 2>&1
-      GOFLAGS="" go ${action} "$1" || install_failed=1
-      popd > /dev/null 2>&1
-    else
-      GOFLAGS="" go ${action} "$1" || install_failed=1
-    fi
+  if [ -z "$(which ${tool})" ]; then
+    local temp_dir="$(mktemp -d)"
+    pushd "${temp_dir}" > /dev/null 2>&1
+    GOFLAGS="" go get "$1" || install_failed=1
+    popd > /dev/null 2>&1
+    rm -rf "${temp_dir}"
   fi
   (( install_failed )) && return ${install_failed}
   shift 2