github/workflows: Cache ESP-IDF checkout and installation.

Cache is keyed on the ESP-IDF version used in CI, so there shouldn't be any
cache invalidation issues when ESP-IDF version changes.

Restoring from cache takes approx 15s, compared to 2-3m to perform these
steps (ESP-IDF tools install, ESP-IDF clone, ESP-IDF submodule clone) the
first time.

Cache size is approx 1.6GB, the git clone is tweaked as much as possible to
keep the size down.

Signed-off-by: Angus Gratton <angus@redyak.com.au>

tools/ci.sh

@@ -118,10 +118,15 @@ function ci_cc3200_build {
 ########################################################################################
 # ports/esp32
 
-function ci_esp32_idf50_setup {
+# GitHub tag of ESP-IDF to use for CI (note: must be a tag or a branch)
+IDF_VER=v5.0.2
+
+function ci_esp32_idf_setup {
     pip3 install pyelftools
-    git clone https://github.com/espressif/esp-idf.git
-    git -C esp-idf checkout v5.0.2
+    git clone --depth 1 --branch $IDF_VER https://github.com/espressif/esp-idf.git
+    # doing a treeless clone isn't quite as good as --shallow-submodules, but it
+    # is smaller than full clones and works when the submodule commit isn't a head.
+    git -C esp-idf submodule update --init --recursive --filter=tree:0
     ./esp-idf/install.sh
 }