Replace #!/bin/bash with #!/usr/bin/env bash to improve compatibility with systems where /bin/bash is not available

tests/benchmarks/comparison.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 if ! which hyperfine > /dev/null 2>&1; then
     echo "'hyperfine' does not seem to be installed."

tests/benchmarks/run-benchmarks.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 if ! which hyperfine > /dev/null 2>&1; then
     echo "'hyperfine' does not seem to be installed."