build.plat: bypass tool detection if NMIGEN_*_env is set.

It's not practical to detect tools within the toolchain environment
for various reasons, so just assume the tools are there if the user
says they are.

Before this commit, the tools would be searched outside the toolchain
environment, which of course would always fail for Vivado, ISE, etc.

nmigen/build/plat.py

@@ -60,12 +60,17 @@ class Platform(ResourceManager, metaclass=ABCMeta):
             raise TypeError("File contents must be str, bytes, or a file-like object")
         self.extra_files[filename] = content
 
+    @property
+    def _toolchain_env_var(self):
+        return f"NMIGEN_{self.toolchain}_env"
+
     def build(self, elaboratable, name="top",
               build_dir="build", do_build=True,
               program_opts=None, do_program=False,
               **kwargs):
-        for tool in self.required_tools:
-            require_tool(tool)
+        if self._toolchain_env_var not in os.environ:
+            for tool in self.required_tools:
+                require_tool(tool)
 
         plan = self.prepare(elaboratable, name, **kwargs)
         if not do_build:
@@ -78,6 +83,8 @@ class Platform(ResourceManager, metaclass=ABCMeta):
         self.toolchain_program(products, name, **(program_opts or {}))
 
     def has_required_tools(self):
+        if self._toolchain_env_var in os.environ:
+            return True
         return all(has_tool(name) for name in self.required_tools)
 
     @abstractmethod
@@ -238,13 +245,13 @@ class TemplatedPlatform(Platform):
         "build_{{name}}.sh": """
             # {{autogenerated}}
             set -e{{verbose("x")}}
-            [ -n "$NMIGEN_{{platform.toolchain}}_env" ] && . "$NMIGEN_{{platform.toolchain}}_env"
+            [ -n "${{platform._toolchain_env_var}}" ] && . "${{platform._toolchain_env_var}}"
             {{emit_commands("sh")}}
         """,
         "build_{{name}}.bat": """
             @rem {{autogenerated}}
             {{quiet("@echo off")}}
-            if defined NMIGEN_{{platform.toolchain}}_env call %NMIGEN_{{platform.toolchain}}_env%
+            if defined {{platform._toolchain_env_var}} call %{{platform._toolchain_env_var}}%
             {{emit_commands("bat")}}
         """,
     }

nmigen/vendor/lattice_ecp5.py

@@ -163,9 +163,9 @@ class LatticeECP5Platform(TemplatedPlatform):
             # {{autogenerated}}
             set -e{{verbose("x")}}
             if [ -z "$BASH" ] ; then exec /bin/bash "$0" "$@"; fi
-            if [ -n "$NMIGEN_{{platform.toolchain}}_env" ]; then
-                bindir=$(dirname "$NMIGEN_{{platform.toolchain}}_env")
-                . "$NMIGEN_{{platform.toolchain}}_env"
+            if [ -n "${{platform._toolchain_env_var}}" ]; then
+                bindir=$(dirname "${{platform._toolchain_env_var}}")
+                . "${{platform._toolchain_env_var}}"
             fi
             {{emit_commands("sh")}}
         """,

nmigen/vendor/xilinx_spartan_3_6.py

@@ -88,7 +88,7 @@ class XilinxSpartan3Or6Platform(TemplatedPlatform):
             # {{autogenerated}}
             set -e{{verbose("x")}}
             if [ -z "$BASH" ] ; then exec /bin/bash "$0" "$@"; fi
-            [ -n "$NMIGEN_{{platform.toolchain}}_env" ] && . "$NMIGEN_{{platform.toolchain}}_env"
+            [ -n "${{platform._toolchain_env_var}}" ] && . "${{platform._toolchain_env_var}}"
             {{emit_commands("sh")}}
         """,
         "{{name}}.v": r"""