Dockerfile: update picolibc to 1.5.1

[benpicco]

This should fix builds with clang.

https://github.com/picolibc/picolibc/releases/tag/1.4.7

https://github.com/picolibc/picolibc/releases/tag/1.5.1

[kaspar030]

travis cannot build picolibc: sh: 0: Can't open ../do-esp32-configure

[benpicco]

Ah turns out the scripts were moved.

[kaspar030]

../meson.build:35:0: ERROR: Unknown compiler(s): [['riscv64-unknown-elf-gcc']]

The follow exceptions were encountered:

Running "riscv64-unknown-elf-gcc --version" gave "[Errno 2] No such file or directory: 'riscv64-unknown-elf-gcc': 'riscv64-unknown-elf-gcc'"

[benpicco]

Yea I thought I could do away with our custom cross-riscv-none-embed.txt while I'm at it, but I guess I was a bit too eager.

[maribu]

With #116 merged this should no longer be needed. If anyone disagrees, please just re-open :)