Add imacros to absolutized cgo env list (#4325)

**What type of PR is this?**
> Bug fix

**What does this PR do? Why is it needed?**
This adds the -imacros flag to the set of cgo compiler flags that are
transformed to absolute paths. Currently relative path values for
-imacros will fail.

**Which issues(s) does this PR fix?**
No existing issue, just seems to be a missed flag.

**Other notes for review**
I have been able to successfully test this locally using my existing cc
toolchain.

Author: LaurenceTews

go/tools/builders/env.go

@@ -35,7 +35,7 @@ var (
 	// cgoEnvVars is the list of all cgo environment variable
 	cgoEnvVars = []string{"CGO_CFLAGS", "CGO_CXXFLAGS", "CGO_CPPFLAGS", "CGO_LDFLAGS"}
 	// cgoAbsEnvFlags are all the flags that need absolute path in cgoEnvVars
-	cgoAbsEnvFlags = []string{"-I", "-L", "-isysroot", "-isystem", "-iquote", "-include", "-gcc-toolchain", "--sysroot", "-resource-dir", "-fsanitize-blacklist", "-fsanitize-ignorelist"}
+	cgoAbsEnvFlags = []string{"-I", "-L", "-isysroot", "-isystem", "-iquote", "-include", "-imacros", "-gcc-toolchain", "--sysroot", "-resource-dir", "-fsanitize-blacklist", "-fsanitize-ignorelist"}
 	// cgoAbsPlaceholder is placed in front of flag values that must be absolutized
 	cgoAbsPlaceholder = "__GO_BAZEL_CC_PLACEHOLDER__"
 )