Support matching release candidate toolchain versions (#3998)

Release candidate versions are of the format `X.YrcZ`.
(ex: golang/go@30b6fd6)

Unfortunately, this format doesn't get matched properly in `go_toolchain.bzl`
so it's not possible to use rc versions without patching `VERSION`,
or you'll run into the error, for e.g., with the added test case:
```
no matching toolchains found for types @io_bazel_rules_go//go:toolchain
```

This PR adds a way to match `rc` versions for the toolchain.

go/private/go_toolchain.bzl

@@ -170,6 +170,14 @@ def declare_bazel_toolchains(
         visibility = ["//visibility:private"],
     )
 
+    native.config_setting(
+        name = prefix + "match_minor_release_candidate",
+        flag_values = {
+            sdk_version_label: major + "." + minor + prerelease,
+        },
+        visibility = ["//visibility:private"],
+    )
+
     native.config_setting(
         name = prefix + "match_sdk_type",
         flag_values = {
@@ -186,6 +194,7 @@ def declare_bazel_toolchains(
             ":" + prefix + "match_major_minor_version",
             ":" + prefix + "match_patch_version",
             ":" + prefix + "match_prerelease_version",
+            ":" + prefix + "match_minor_release_candidate",
             ":" + prefix + "match_sdk_type",
         ],
         visibility = ["//visibility:private"],

tests/core/cross/sdk_version_test.go

@@ -53,6 +53,11 @@ var testCases = []testcase{
 		SDKVersion:      "1.17.1",
 		expectedVersion: "go1.17.1",
 	},
+	{
+		Name:             "1_17_release_candidate",
+		SDKVersion:       "1.17rc1",
+		expectedVersion:  "go1.17rc1",
+	},
 }
 
 func TestMain(m *testing.M) {
@@ -79,6 +84,11 @@ go_download_sdk(
     name = "go_sdk_1_17_1",
     version = "1.17.1",
 )
+go_download_sdk(
+    name = "go_sdk_1_17_rc1",
+    version = "1.17rc1",
+)
+
 go_register_toolchains()
 -- main.go --
 package main