sh: Use StringBuilder instead of C-like manual joining

apps/sh.cpp

@@ -1,3 +1,4 @@
+#include <luna/StringBuilder.h>
 #include <luna/Vector.h>
 
 #include <errno.h>
@@ -30,21 +31,23 @@ static Result<Vector<char*>> split_command_into_argv(const char* cmd)
     return result;
 }
 
-static char* join_path(const char* str1, const char* str2)
+static Result<String> join_path(const char* str1, const char* str2)
 {
-    char* buf = (char*)malloc(strlen(str1) + strlen(str2) + 1);
-    strlcpy(buf, str1, strlen(str1) + 1);
-    strncat(buf, str2, strlen(str2));
-    return buf;
+    StringBuilder sb;
+    TRY(sb.add(StringView { str1 }));
+    TRY(sb.add(StringView { str2 }));
+    return sb.string();
 }
 
 static int execute_in_path(const char* dir, char** argv)
 {
-    char* path = join_path(dir, argv[0]);
+    String path;
+    bool ok = join_path(dir, argv[0]).try_move_value_or_error(path, errno);
+    if (!ok) return -1;
+
     int err = errno;
 
-    int status = execv(path, argv);
-    free(path);
+    int status = execv(path.chars(), argv);
 
     if (errno == ENOENT)
     {