[vcpkg] Allow to use Nuget's cache for Nuget binary caching sources (fix #15169) (#15512)

* Fix warning on clang version 10.0.0-4ubuntu1

The warning was

```shell
../src/vcpkg/commands.porthistory.cpp:55:14: error: unused function 'is_date' [-Werror,-Wunused-function]
```

* Add environment variable VCPKG_USE_NUGET_CACHE

As the name suggests, this environment variable allow tu use Nuget
cache for Nuget binary caching sources.

* Document NuGet's Cache environment variable

src/vcpkg/binarycaching.cpp

@@ -2,6 +2,7 @@
 #include <vcpkg/base/downloads.h>
 #include <vcpkg/base/files.h>
 #include <vcpkg/base/parse.h>
+#include <vcpkg/base/strings.h>
 #include <vcpkg/base/system.debug.h>
 #include <vcpkg/base/system.print.h>
 #include <vcpkg/base/system.process.h>
@@ -415,7 +416,11 @@ namespace
             , m_read_configs(std::move(read_configs))
             , m_write_configs(std::move(write_configs))
             , m_interactive(interactive)
+            , m_use_nuget_cache(false)
         {
+            const std::string use_nuget_cache = System::get_environment_variable("VCPKG_USE_NUGET_CACHE").value_or("");
+            m_use_nuget_cache = Strings::case_insensitive_ascii_equals(use_nuget_cache, "true") ||
+                                Strings::case_insensitive_ascii_equals(use_nuget_cache, "1");
         }
 
         int run_nuget_commandline(const System::Command& cmdline)
@@ -531,9 +536,7 @@ namespace
                     .string_arg("-Source")
                     .string_arg(Strings::join(";", m_read_sources))
                     .string_arg("-ExcludeVersion")
-                    .string_arg("-NoCache")
                     .string_arg("-PreRelease")
-                    .string_arg("-DirectDownload")
                     .string_arg("-PackageSaveMode")
                     .string_arg("nupkg")
                     .string_arg("-Verbosity")
@@ -543,6 +546,10 @@ namespace
                 {
                     cmdline.string_arg("-NonInteractive");
                 }
+                if (!m_use_nuget_cache)
+                {
+                    cmdline.string_arg("-DirectDownload").string_arg("-NoCache");
+                }
 
                 cmdlines.push_back(std::move(cmdline));
             }
@@ -561,9 +568,7 @@ namespace
                     .string_arg("-ConfigFile")
                     .path_arg(cfg)
                     .string_arg("-ExcludeVersion")
-                    .string_arg("-NoCache")
                     .string_arg("-PreRelease")
-                    .string_arg("-DirectDownload")
                     .string_arg("-PackageSaveMode")
                     .string_arg("nupkg")
                     .string_arg("-Verbosity")
@@ -573,6 +578,10 @@ namespace
                 {
                     cmdline.string_arg("-NonInteractive");
                 }
+                if (!m_use_nuget_cache)
+                {
+                    cmdline.string_arg("-DirectDownload").string_arg("-NoCache");
+                }
 
                 cmdlines.push_back(std::move(cmdline));
             }
@@ -735,6 +744,7 @@ namespace
 
         std::set<PackageSpec> m_restored;
         bool m_interactive;
+        bool m_use_nuget_cache;
     };
 }
 
@@ -1429,6 +1439,9 @@ void vcpkg::help_topic_binary_caching(const VcpkgPaths&)
              "\n"
              "if the appropriate environment variables are defined and non-empty.\n");
     tbl.blank();
+    tbl.text("NuGet's cache is not used by default. To use it for every nuget-based source, set the environment "
+             "variable `VCPKG_USE_NUGET_CACHE` to `true` (case-insensitive) or `1`.\n");
+    tbl.blank();
     System::print2(tbl.m_str);
     const auto& maybe_cachepath = default_cache_path();
     if (auto p = maybe_cachepath.get())