ci: 冷机上目标 C 库是惰性安装的,构建要跑两遍

三个仓库的失败共享同一条根因,而报错都指向头文件、没有一处指向真正
没发生的那件事:

    inttypes.h:24:15: fatal error: 'inttypes.h' file not found
    riscv_virt_rt.cppm:9:10: fatal error: 'stdio.h' file not found

mcpp 是在**构建过程中**从目标行安装目标 C 库的。于是一台从没为这个
triple 构建过的机器,第一遍就会去编译需要那些头的源码,而它们还不存在;
第二遍就有了。mcpp 自己对同类情形的措辞是「expected on the first build
in a fresh MCPP_HOME; a second build resolves it」。

所以预热是**两遍,第一遍允许失败**。⚠️ 刻意不用
\`xlings install xim:picolibc-riscv\`:那会让目标世界绕过引擎自己的安装
而存在,而那条安装出回归时,这些仓库正好就再也发现不了。预热构建走的是
它,手工安装则是替换掉它。

portability 矩阵的每一行都是一台从没构建过这个目标的机器,所以那里同样
要两遍——单遍时它报出来的「可移植性失败」其实是冷机失败。

Author: Sunrisepeak

.github/workflows/ci.yml

@@ -172,4 +172,11 @@ jobs:
 
       - name: The backend cross-builds
         run: |
+          # ⚠️ TWICE, AND THE FIRST IS ALLOWED TO FAIL — every row of this
+          # matrix is a machine that has never targeted this triple, which is
+          # precisely where mcpp's lazy install of the target C library shows.
+          # The first build compiles sources needing its headers before they
+          # exist; the second has them. A single build here reported a
+          # portability failure that was really a cold-machine one.
+          mcpp build --target riscv64-none-elf || true
           mcpp build --target riscv64-none-elf