Fix #1161: set the CMAKE environment variables for prebuilt toolcha…

…ins.

This commit is an attempt to fix #1161 by setting the `CMAKE` environment
variable (through [`TemplateVariableInfo`]).

[`TemplateVariableInfo`]: https://bazel.build/rules/lib/providers/TemplateVariableInfo

Since the CMake prebuilt packages yield multiple files, we cannot use `$(execpath)`
to get the path to the CMake binary, as we do for Ninja:

https://github.com/bazelbuild/rules_foreign_cc/blob/c2e097455d2bbf92b2ae71611d1261ba79eb8aa8/toolchains/prebuilt_toolchains.py#L449

To circumvent this issue, this commit adds a new attribute called `tools` to
the [`native_tool_toolchain`] rule. Through this attribute, extra targets
can be passed to [`_resolve_tool_path`] in order to be able to expand the path
to a single file using `$(execpath)`.

[`native_tool_toolchain`]: https://github.com/bazelbuild/rules_foreign_cc/blob/c2e097455d2bbf92b2ae71611d1261ba79eb8aa8/toolchains/native_tools/native_tools_toolchain.bzl#L40
[`_resolve_tool_path`]: https://github.com/bazelbuild/rules_foreign_cc/blob/c2e097455d2bbf92b2ae71611d1261ba79eb8aa8/toolchains/native_tools/native_tools_toolchain.bzl#L22

toolchains/native_tools/native_tools_toolchain.bzl

@@ -19,7 +19,7 @@ ToolInfo = provider(
     },
 )
 
-def _resolve_tool_path(ctx, path, target):
+def _resolve_tool_path(ctx, path, target, tools):
     """
         Resolve the path to a tool.
 
@@ -32,7 +32,7 @@ def _resolve_tool_path(ctx, path, target):
     _, resolved_bash_command, _ = ctx.resolve_command(
         command = path,
         expand_locations = True,
-        tools = [target],
+        tools = tools + [target],
     )
 
     return resolved_bash_command[-1]
@@ -43,10 +43,10 @@ def _native_tool_toolchain_impl(ctx):
     path = None
     env = {}
     if ctx.attr.target:
-        path = _resolve_tool_path(ctx, ctx.attr.path, ctx.attr.target)
+        path = _resolve_tool_path(ctx, ctx.attr.path, ctx.attr.target, ctx.attr.tools)
 
         for k, v in ctx.attr.env.items():
-            env[k] = _resolve_tool_path(ctx, v, ctx.attr.target)
+            env[k] = _resolve_tool_path(ctx, v, ctx.attr.target, ctx.attr.tools)
 
     else:
         path = ctx.expand_location(ctx.attr.path)
@@ -88,6 +88,16 @@ native_tool_toolchain = rule(
             ),
             allow_files = True,
         ),
+        "tools": attr.label_list(
+            mandatory = False,
+            cfg = "exec",
+            doc = (
+                "Additional tools." +
+                "If `target` expands to several files, `tools` can be used to " +
+                "isolate a specific file that can be used in `env`."
+            ),
+            allow_files = True,
+        ),
     },
     incompatible_use_toolchain_transition = True,
 )