no-change gclient sync/build should be no-op, but now builds many targets

[aam]

PS C:\src\d\sdk> $env:RBE=1; python3 tools\build.py -ax64 -mrelease dart runtime
Generating Visual Studio projects took 219ms
Done. Made 607 targets from 131 files in 1049ms
Proxy started successfully.
buildtools/ninja/ninja -C out\ReleaseX64 -j200 -l64 dart runtime
ninja: Entering directory `out\ReleaseX64'
ninja: no work to do.
buildtools/ninja/ninja -C out\ReleaseX64 -j200 -l64 dart runtime done.
The build took 3.163 seconds
PS C:\src\d\sdk> gclient sync -D
Updating depot_tools...
Syncing projects: 100% (61/61), done.
Running hooks: 100% (9/9), done.
+===================================================+
|      METRICS COLLECTION IS TAKING PLACE           |
|                                                   |
| To suppress this message opt in or out using:     |
| $ gclient metrics [--opt-in] [--opt-out]          |
| For more information please see metrics.README.md |
| in your depot_tools checkout or visit             |
| https://bit.ly/3MpLAYM.                           |
+===================================================+
PS C:\src\d\sdk> $env:RBE=1; python3 tools\build.py -ax64 -mrelease dart runtime
Generating Visual Studio projects took 199ms
Done. Made 607 targets from 131 files in 1064ms
Proxy started successfully.
buildtools/ninja/ninja -C out\ReleaseX64 -j200 -l64 dart runtime
ninja: Entering directory `out\ReleaseX64'
[0/56] ACTION //runtime/vm:vm_platform(//build/toolchain/win:clang_x64_shared)

This started with b9b7705 (reverted, then relanded later) which updated sdk/tools/generate_package_config.dart

cc @sigurdm

[sigurdm]

Not sure what is causing this

cc @athomas

[sigurdm]

Most likely (suggested by @athomas) it is because pub writes a timestamp in the package_config file that triggers an update all the way through.

[sigurdm]

It is not (only) the timestamp in the file. But the modification of the file itself that triggers ninja to rebuild stuff