do not create ./build in cwd when cache init is skipped (#1035)

sharedCache is allocated before goVersionOK runs, but CacheDir is
only set once the version check passes. The trim defer in mainErr
only checked sharedCache != nil, so on the too-new-version path
openCache joined an empty CacheDir with "build" and created the
directory in CWD.

Fixes #995.

main.go

@@ -261,7 +261,7 @@ func mainErr(args []string) error {
 				fmt.Fprintf(os.Stderr, "could not clean up GARBLE_SHARED: %v\n", err)
 			}
 			// skip the trim if we didn't even start a build
-			if sharedCache != nil {
+			if sharedCache != nil && sharedCache.CacheDir != "" {
 				fsCache, err := openCache()
 				if err == nil {
 					err = fsCache.Trim()

testdata/script/goversion.txtar

@@ -39,6 +39,8 @@ env GARBLE_TEST_GOVERSION='go1.38.2'
 env TOOLCHAIN_GOVERSION='go1.38.2'
 ! exec garble build
 stderr 'Go version "go1\.38\.2" is too new; Go linker patches aren''t available for go1\.27 or later yet'
+# Ensure we don't create a cache directory in the current dir.
+! exists build
 
 # We should accept custom devel strings.
 env TOOLCHAIN_GOVERSION='go1.26.0-somecustomversion'