sqlite: use bundled amalgamation on Linux/macOS when pkg-config is unavailable

[quaesitor-scientiam]

Summary

• When pkg-config sqlite3 fails on Linux/macOS, the module previously fell back to -lsqlite3, which requires a system development package to be installed
• This PR replaces that fallback with #flag @VEXEROOT/thirdparty/sqlite/sqlite3.c, compiling the bundled amalgamation directly — consistent with what already works on Windows
• Users without a system SQLite package can run vlib/db/sqlite/install_thirdparty_sqlite.vsh to obtain the amalgamation

Test plan

• Run vlib/db/sqlite/install_thirdparty_sqlite.vsh on a Linux system without libsqlite3-dev installed
• Verify v run vlib/db/sqlite/sqlite_test.v passes

Fixes #26830

[quaesitor-scientiam]

All the failures are unrelated to this change. The common failure across all Linux CI jobs is:

native error: vlib/v/gen/native/gen.v:1218, &native.Gen{}.patch_calls fn addr of `__at_least_one` = 0

This is a pre-existing bug in the native backend (-b native) affecting vlib/v/gen/native/tests/native_test.v. The CI installs libsqlite3-dev itself, so it hits the pkg-config branch and this change is never even exercised.

The mssql and mysql failures are also unrelated — the mssql log shows the SQL Server container initializing (a timeout/infrastructure issue).

The docker-alpine-musl jobs are likely also the same native backend issue. This change is clean — the failures are pre-existing on master.