[CI] change ci/install_dependencies shebang from bash to sh

- the script needs to be able to bootstrap everything even on platforms where we
  don't have bash
- drop function which is bash specific

(cherry picked from commit 9f01b86d2219d2f4cbb23730bba79c2b779621a2)

ci/install_dependencies.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/bin/sh
 
 set -e
 
@@ -7,14 +7,13 @@ set -e
 #
 # This script builds a shared library of googletest (not googlemock!) inside
 # gtest_build and copies it to /usr/lib/
-function debian_build_gtest() {
-    pushd .
+debian_build_gtest() {
     [ -d gtest_build ] || mkdir gtest_build
     cd gtest_build
     cmake -DBUILD_SHARED_LIBS=1 /usr/src/googletest/googletest
     make
     cp libgtest* /usr/lib/
-    popd
+    cd ..
 }
 
 distro_id=$(grep '^ID=' /etc/os-release|awk -F = '{print $2}'|sed 's/\"//g')