Make eng/build.sh portable (don't hardcode /bin/bash) (#14707)

Follow-up to #14361 addressing @corngood's review feedback:
`eng/build.sh` isn't portable and breaks on NixOS.

- Shebang `#! /bin/bash` -> `#!/usr/bin/env bash`, matching every other
script in this repo (including `build.sh` and `eng/common/*.sh`).
- The hardcoded `/bin/bash` invocations of the stage 1 / stage 2 builds
now use `bash` from `\`. The echoed command lines were updated to match.

No behavioral change on distributions where bash lives at `/bin/bash`.

Copilot-Session: fcd4cd36-f7ca-48cf-9c4d-7f9601ef3db2

Author: ViktorHofer

eng/build.sh

@@ -1,4 +1,4 @@
-#! /bin/bash
+#!/usr/bin/env bash
 
 configuration="Debug"
 test=false
@@ -92,10 +92,10 @@ fi
 
 # Log the stage 1 build command so that it's clear which arguments flow to it.
 if [ "$stage2" = true ]; then
-  echo "Stage 1 build: /bin/bash \"$build_script\" ${build_args[*]}"
+  printf 'Stage 1 build: %q %q %s\n' "$BASH" "$build_script" "$(printf '%q ' "${build_args[@]}")"
 fi
 
-/bin/bash "$build_script" "${build_args[@]}"
+"$BASH" "$build_script" "${build_args[@]}"
 stage1_exit_code=$?
 
 if [ "$stage2" != true ]; then
@@ -174,7 +174,7 @@ fi
 # until a stable version of MT is available in the images.
 stage2_build_args+=( ${stage2Arguments[@]+"${stage2Arguments[@]}"} )
 
-echo "Stage 2 build: /bin/bash \"$build_script\" ${stage2_build_args[*]}"
+printf 'Stage 2 build: %q %q %s\n' "$BASH" "$build_script" "$(printf '%q ' "${stage2_build_args[@]}")"
 # Needs to run out-of-proc to not inherit the stage 1 build's state variables.
-/bin/bash "$build_script" "${stage2_build_args[@]}"
+"$BASH" "$build_script" "${stage2_build_args[@]}"
 exit $?