install_python(): discover brew from IDE

On macOS, in the RStudio IDE, brew might not be on the PATH, even if it's avilable on the system.
This patch causes reticulate to look for brew in the default homebrew install location, and temporarily add it to the PATH for pyenv to find.

R/pyenv.R

@@ -157,6 +157,11 @@ pyenv_install <- function(version, force, pyenv = NULL) {
   # set options
   withr::local_envvar(PYTHON_CONFIGURE_OPTS = "--enable-shared")
 
+  if(is_macos() &&
+     Sys.which("brew") == "" &&
+     file.exists("/opt/homebrew/bin/brew"))
+    withr::local_path("/opt/homebrew/bin")
+
   # build install arguments
   force <- if (force)
     "--force"
@@ -221,9 +226,15 @@ pyenv_bootstrap_unix <- function() {
   # pyenv python builds are substantially faster on macOS if we pre-install
   # some dependencies (especially openssl) as pre-built but "untapped kegs"
   # (i.e., unlinked to somewhere on the PATH but tucked away under $BREW_ROOT/Cellar).
-  if (nzchar(Sys.which("brew"))) {
-    system2t("brew", c("install -q openssl readline sqlite3 xz zlib tcl-tk"))
-    system2t("brew", c("install --only-dependencies pyenv python"))
+  if (is_macos()) {
+    brew <- Sys.which("brew")
+    if(brew == "" && file.exists(brew <- "/opt/homebrew/bin/brew"))
+      withr::local_path("/opt/homebrew/bin")
+
+    if(file.exists(brew)) {
+      system2t(brew, c("install -q openssl readline sqlite3 xz zlib tcl-tk"))
+      system2t(brew, c("install --only-dependencies pyenv python"))
+    }
   }
 
   # download the installer