build.plat,vendor: start build.sh with #!/bin/sh.

The build scripts generated by Amaranth are designed to be invoked by
directly running them with any shell (some of them will re-invoke
themselves with `bash` specifically, when it's a toolchain requirement),
and they're not currently marked executable, so there's no shebang.

Add a shebang line to improve compatibility with cases where they are
treated as executables in their own right.

amaranth/build/plat.py

@@ -276,6 +276,7 @@ class TemplatedPlatform(Platform):
 
     build_script_templates = {
         "build_{{name}}.sh": """
+            #!/bin/sh
             # {{autogenerated}}
             set -e{{verbose("x")}}
             {% for var in platform._all_toolchain_env_vars %}

amaranth/vendor/_intel.py

@@ -80,6 +80,7 @@ class IntelPlatform(TemplatedPlatform):
     quartus_file_templates = {
         **TemplatedPlatform.build_script_templates,
         "build_{{name}}.sh": r"""
+            #!/bin/sh
             # {{autogenerated}}
             {% for var in platform._all_toolchain_env_vars %}
             if [ -n "${{var}}" ]; then

amaranth/vendor/_lattice_ecp5.py

@@ -182,6 +182,7 @@ class LatticeECP5Platform(TemplatedPlatform):
     _diamond_file_templates = {
         **TemplatedPlatform.build_script_templates,
         "build_{{name}}.sh": r"""
+            #!/bin/sh
             # {{autogenerated}}
             set -e{{verbose("x")}}
             if [ -z "$BASH" ] ; then exec /bin/bash "$0" "$@"; fi

amaranth/vendor/_lattice_ice40.py

@@ -175,6 +175,7 @@ class LatticeICE40Platform(TemplatedPlatform):
     _icecube2_file_templates = {
         **TemplatedPlatform.build_script_templates,
         "build_{{name}}.sh": r"""
+            #!/bin/sh
             # {{autogenerated}}
             set -e{{verbose("x")}}
             {% for var in platform._all_toolchain_env_vars %}

amaranth/vendor/_xilinx.py

@@ -139,6 +139,7 @@ class XilinxPlatform(TemplatedPlatform):
     _vivado_file_templates = {
         **TemplatedPlatform.build_script_templates,
         "build_{{name}}.sh": r"""
+            #!/bin/sh
             # {{autogenerated}}
             set -e{{verbose("x")}}
             if [ -z "$BASH" ] ; then exec /bin/bash "$0" "$@"; fi