Update platformio.ini : fix on IDF 3.4.0 on ESP32

The code of this project is compatible with the different versions of IDF. So in case you want to port the software on more recent chips (C3/S2) things should work ok.

The implementation of I2S on the original ESP32 chip however contains more and more bugs in recent IDF 4.x.x versions, such as
- I2S mic input not working
- DAC output not working
- adc input not working

So we fix IDF version to 3.4.0 until the issues are fixed.

platformio.ini

@@ -15,7 +15,6 @@ extra_configs =
 
 [env]
 framework = arduino
-platform = espressif32
 ; upload_port = /dev/cu.SLAB_USBtoUART
 ; monitor_port = /dev/cu.SLAB_USBtoUART
 monitor_speed = 115200
@@ -24,10 +23,12 @@ build_flags = -Ofast
 
 [env:tinypico]
 board = tinypico
+platform = [email protected]
 lib_deps = tinypico/TinyPICO Helper Library@^1.4.0
 build_flags = -Ofast -D USE_I2S_MIC_INPUT -D USE_ESP_NOW
 
 [env:lolin32]
 board = lolin32
+platform = [email protected]
 build_flags = -Ofast -D USE_I2S_MIC_INPUT -D USE_ESP_NOW
 lib_ignore = indicator_led_pico