Add sysroot variable to cc_common.create_link_variables

Before this cl the sysroot variable was not present, and that's a bug.

RELNOTES: None
PiperOrigin-RevId: 203346557
diff --git a/src/main/java/com/google/devtools/build/lib/rules/cpp/LinkBuildVariables.java b/src/main/java/com/google/devtools/build/lib/rules/cpp/LinkBuildVariables.java
index 0868b20c..63a4b48 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/cpp/LinkBuildVariables.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/cpp/LinkBuildVariables.java
@@ -123,7 +123,8 @@
       boolean isLegacyFullyStaticLinkingMode,
       boolean isStaticLinkingMode)
       throws EvalException {
-    CcToolchainVariables.Builder buildVariables = new CcToolchainVariables.Builder();
+    CcToolchainVariables.Builder buildVariables =
+        new CcToolchainVariables.Builder(ccToolchainProvider.getBuildVariables());
 
     // symbol counting
     if (symbolCounts != null) {
diff --git a/src/test/java/com/google/devtools/build/lib/rules/cpp/SkylarkCcCommonTest.java b/src/test/java/com/google/devtools/build/lib/rules/cpp/SkylarkCcCommonTest.java
index 3d4a6d5..d52b0c4 100644
--- a/src/test/java/com/google/devtools/build/lib/rules/cpp/SkylarkCcCommonTest.java
+++ b/src/test/java/com/google/devtools/build/lib/rules/cpp/SkylarkCcCommonTest.java
@@ -503,6 +503,35 @@
   }
 
   @Test
+  public void testEmptyLinkVariablesContainSysroot() throws Exception {
+    AnalysisMock.get()
+        .ccSupport()
+        .setupCrosstool(
+            mockToolsConfig,
+            "builtin_sysroot: '/foo/bar/sysroot'",
+            "feature {",
+            "  name: 'sysroot'",
+            "  enabled: true",
+            "  flag_set {",
+            "    action: 'c++-link-executable'",
+            "    flag_group {",
+            "      expand_if_all_available: 'sysroot'",
+            "      flag: '--yolo_sysroot_flag=%{sysroot}'",
+            "    }",
+            "  }",
+            "}");
+    useConfiguration();
+    assertThat(
+            commandLineForVariables(
+                CppActionNames.CPP_LINK_EXECUTABLE,
+                "cc_common.create_link_variables(",
+                "feature_configuration = feature_configuration,",
+                "cc_toolchain = toolchain,",
+                ")"))
+        .contains("--yolo_sysroot_flag=/foo/bar/sysroot");
+  }
+
+  @Test
   public void testLibrarySearchDirectoriesLinkVariables() throws Exception {
     AnalysisMock.get()
         .ccSupport()