Do not assume FHS in scripts

On systems that do not follow the Filesystem Hierarchy Standard, such as
guix, the hardcoded `/bin/pwd` will fail to be found so that the script
will fail.

Use `pwd`, instead, so that the command can be found through the normal
path search mechanism.

See https://github.com/qt/qtbase/commit/3388de698bfb9bbc456c08f03e83bf3e749df35c.

diff --git a/qtbase/configure b/qtbase/configure
index 08b49a8d..faea5b55 100755
--- a/qtbase/configure
+++ b/qtbase/configure
@@ -9,9 +9,9 @@
 
 # the directory of this script is the "source tree"
 relpath=`dirname "$0"`
-relpath=`(cd "$relpath"; /bin/pwd)`
+relpath=`(cd "$relpath"; pwd)`
 # the current directory is the "build tree" or "object tree"
-outpath=`/bin/pwd`
+outpath=`pwd`
 outpathPrefix=$outpath
 
 # do this early so we don't store it in config.status

--- a/qtbase/bin/qt-cmake.in
+++ b/qtbase/bin/qt-cmake.in
@@ -2,7 +2,7 @@
 
 # The directory of this script is the expanded absolute path of the "$qt_prefix/bin" directory.
 script_dir_path=`dirname $0`
-script_dir_path=`(cd "$script_dir_path"; /bin/pwd)`
+script_dir_path=`(cd "$script_dir_path"; pwd)`
 
 # Try to use original cmake, otherwise to make it relocatable, use any cmake found in PATH.
 original_cmake_path="@CMAKE_COMMAND@"

--- a/qtbase/bin/qt-configure-module.in
+++ b/qtbase/bin/qt-configure-module.in
@@ -2,7 +2,7 @@
 set -eu
 
 script_dir_path=`dirname $0`
-script_dir_path=`(cd "$script_dir_path"; /bin/pwd)`
+script_dir_path=`(cd "$script_dir_path"; pwd)`
 
 printUsage()
 {