Use /usr/bin/env shebang - /bin/bash is not universal

On FreeBSD bash is /usr/local/bin/bash.

c++/regenerate-bootstraps.sh

-#! /bin/bash
+#! /usr/bin/env bash
 
 set -euo pipefail
 

c++/samples/test.sh

-#! /bin/bash
+#! /usr/bin/env bash
 #
 # Quick script that compiles and runs the samples, then cleans up.
 # Used for release testing.

c++/setup-autotools.sh

-#! /bin/bash
+#! /usr/bin/env bash
 
 set -euo pipefail
 

c++/setup-ekam.sh

-#! /bin/bash
+#! /usr/bin/env bash
 
 set -euo pipefail
 

doc/push-site.sh

-#! /bin/bash
+#! /usr/bin/env bash
 
 set -eu
 shopt -s extglob

release.sh

-#! /bin/bash
+#! /usr/bin/env bash
 
 set -euo pipefail
 

super-test.sh

-#! /bin/bash
+#! /usr/bin/env bash
 
 set -euo pipefail