GLOB importation doesn't work with folders

[AnthonyGrondin]

When using the GLOB environment variables, the documentation specifies that you can import folders, using the following syntax:

ESP_IDF_GLOB_HOMEDIR_BASE = "/home/someuser"
ESP_IDF_GLOB_HOMEDIR_FOO = "foo*"
ESP_IDF_GLOB_HOMEDIR_BAR = "bar*"

but when using folders as the glob mask, the function that copies files over panics
https://github.com/esp-rs/embuild/blob/5e12da302f372c05d20a149b00eca1befde185f6/src/fs.rs#L14-L21

Example

# .cargo/config.toml
[env]
ESP_IDF_GLOB_BASE = { value = ".", relative = true}
ESP_IDF_GLOB_CERT = "certs*"

.
├── build.rs
├── Cargo.lock
├── Cargo.toml
├── certs/
├── README.md
├── rust-toolchain.toml
├── sdkconfig.defaults
└── src
     └── main.rs

Error log:

Caused by:
  process didn't exit successfully: `/home/esp/project/target/debug/build/esp-idf-sys-cb4238fce024232d/build-script-build` (exit status: 101)
  --- stdout
  cargo:rerun-if-env-changed=ESP_IDF_TOOLS_INSTALL_DIR
  cargo:rerun-if-env-changed=ESP_IDF_SDKCONFIG
  cargo:rerun-if-env-changed=ESP_IDF_SDKCONFIG_DEFAULTS
  cargo:rerun-if-env-changed=MCU
  cargo:rerun-if-env-changed=ESP_IDF_SYS_ROOT_CRATE
  cargo:rerun-if-env-changed=ESP_IDF_VERSION
  cargo:rerun-if-env-changed=ESP_IDF_REPOSITORY
  cargo:rerun-if-env-changed=ESP_IDF_CMAKE_GENERATOR
  cargo:rerun-if-env-changed=IDF_PATH
  cargo:rerun-if-env-changed=ESP_IDF_COMPONENTS
  cargo:rerun-if-changed=/home/esp/project/./certs

  --- stderr
  Build configuration: BuildConfig {
      esp_idf_tools_install_dir: None,
      esp_idf_sdkconfig: None,
      esp_idf_sdkconfig_defaults: None,
      mcu: None,
      native: NativeConfig {
          esp_idf_version: Some(
              Branch(
                  "release/v4.4",
              ),
          ),
          esp_idf_repository: None,
          esp_idf_cmake_generator: None,
          idf_path: Some(
              "/home/esp/.espressif/frameworks/esp-idf-release-v4.4",
          ),
          extra_components: [],
          esp_idf_components: None,
      },
      esp_idf_sys_root_crate: None,
  }
  Using activated esp-idf v4.4.2 environment at '/home/esp/.espressif/frameworks/esp-idf-release-v4.4'
  thread 'main' panicked at 'assertion failed: src_file.is_file()', /home/esp/.cargo/registry/src/github.com-1ecc6299db9ec823/embuild-0.30.3/src/fs.rs:21:5
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace