Add shebang onto the regression script. (#292)

The fuzzing script, as it stands right now, works fine if run directly,
but can encounter issues if used with `--run_under`. By adding a shebang
at the top, it can be used more flexibly.

fuzzing/private/common.bzl

diff --git a/fuzzing/private/common.bzl b/fuzzing/private/common.bzl
index c534692..8f072ed 100644
--- a/fuzzing/private/common.bzl
+++ b/fuzzing/private/common.bzl
@@ -20,7 +20,7 @@
     binary_info = ctx.attr.binary[FuzzingBinaryInfo]
     script = ctx.actions.declare_file(ctx.label.name)
 
-    script_template = """
+    script_template = """#!/usr/bin/env bash
 {environment}
 echo "Launching {binary_path} as a {engine_name} fuzz test..."
 RUNFILES_DIR="$0.runfiles" \

fuzzing/private/regression.bzl

diff --git a/fuzzing/private/regression.bzl b/fuzzing/private/regression.bzl
index 6894267..333dd03 100644
--- a/fuzzing/private/regression.bzl
+++ b/fuzzing/private/regression.bzl
@@ -19,7 +19,7 @@
 def _fuzzing_regression_test_impl(ctx):
     binary_info = ctx.attr.binary[FuzzingBinaryInfo]
     script = ctx.actions.declare_file(ctx.label.name)
-    script_template = """
+    script_template = """#!/usr/bin/env bash
 export FUZZER_OUTPUT_CORPUS_DIR="$TEST_TMPDIR/corpus"
 export FUZZER_ARTIFACTS_DIR="$TEST_TMPDIR/artifacts"
 export FUZZER_BINARY='{fuzzer_binary}'